Add sourcemap support for onLoad plugins

This implements full sourcemap support for Bun's bundler onLoad plugins, allowing plugins to provide sourcemaps that preserve original source content in the final output.

Key changes:
- Added `sourcemap` parameter to onLoad plugin interface (string  < /dev/null |  Uint8Array)
- Updated C++ bindings to pass sourcemap from JS to Zig
- Extended InputFile structure to store parsed sourcemaps and original source references
- Enhanced sourcemap parsing to preserve sources_content when requested
- Modified output sourcemap generation to use original source content from input sourcemaps
- Added comprehensive test suite covering basic, TypeScript-like, and remapping scenarios

The implementation correctly handles:
- Parsing input sourcemaps with sources_content preservation
- Creating virtual InputFile entries for original source content
- Remapping source locations through input sourcemaps during output generation
- Proper memory management for sourcemap data

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/sourcemap/sourcemap.zig

@@ -35,6 +35,8 @@ pub const ParseUrlResultHint = union(enum) {
         column: i32,
         include_names: bool = false,
     },
+    /// Parse mappings and also store sources_content in the ParsedSourceMap
+    mappings_and_source_content,
 };
 
 pub const ParseUrl = struct {
@@ -210,7 +212,19 @@ pub fn parseJSON(
 
         const ptr = bun.new(ParsedSourceMap, map_data);
         ptr.external_source_names = source_paths_slice.?;
-
+        // Store sources_content if requested
+        if (hint == .mappings_and_source_content) {
+            const content_slice = alloc.alloc([]const u8, sources_content.items.len) catch bun.outOfMemory();
+            for (sources_content.items.slice(), 0..) |item, idx| {
+                if (item.data != .e_string) {
+                    content_slice[idx] = "";
+                } else {
+                    content_slice[idx] = alloc.dupe(u8, item.data.e_string.string(alloc) catch "") catch bun.outOfMemory();
+                }
+            }
+            ptr.sources_content = content_slice;
+        }
+        
         break :map ptr;
     } else null;
     errdefer if (map) |m| m.deref();
@@ -223,9 +237,10 @@ pub fn parseJSON(
             break :brk .{ mapping, std.math.cast(u32, mapping.source_index) };
         },
         .mappings_only => .{ null, null },
+        .mappings_and_source_content => .{ null, null },
     };
 
-    const content_slice: ?[]const u8 = if (hint != .mappings_only and
+    const content_slice: ?[]const u8 = if (hint != .mappings_only and hint != .mappings_and_source_content and
         source_index != null and
         source_index.? < sources_content.items.len)
     content: {
@@ -876,6 +891,9 @@ pub const ParsedSourceMap = struct {
     /// loaded without transpilation but with external sources. This array
     /// maps `source_index` to the correct filename.
     external_source_names: []const []const u8 = &.{},
+    /// Sources content from the original sourcemap, indexed the same as external_source_names
+    /// Only populated when parsing sourcemaps from onLoad plugins
+    sources_content: []const []const u8 = &.{},
     /// In order to load source contents from a source-map after the fact,
     /// a handle to the underlying source provider is stored. Within this pointer,
     /// a flag is stored if it is known to be an inline or external source map.
@@ -951,6 +969,12 @@ pub const ParsedSourceMap = struct {
             allocator.free(this.external_source_names);
         }
 
+        if (this.sources_content.len > 0) {
+            for (this.sources_content) |content|
+                allocator.free(content);
+            allocator.free(this.sources_content);
+        }
+
         bun.destroy(this);
     }
 
@@ -1764,6 +1788,7 @@ pub const Chunk = struct {
     pub fn NewBuilder(comptime SourceMapFormatType: type) type {
         return struct {
             const ThisBuilder = @This();
+            input_source_map: ?*ParsedSourceMap = null,
             source_map: SourceMapper,
             line_offset_tables: LineOffsetTable.List = .{},
             prev_state: SourceMapState = SourceMapState{},
@@ -1877,7 +1902,17 @@ pub const Chunk = struct {
                 b.last_generated_update = @as(u32, @truncate(output.len));
             }
 
-            pub fn appendMapping(b: *ThisBuilder, current_state: SourceMapState) void {
+            pub fn appendMapping(b: *ThisBuilder, current_state_: SourceMapState) void {
+                var current_state = current_state_;
+                // If the input file had a source map, map all the way back to the original
+                if (b.input_source_map) |input| {
+                    if (Mapping.find(input.mappings, current_state.original_line, current_state.original_column)) |mapping| {
+                        current_state.source_index = mapping.source_index;
+                        current_state.original_line = mapping.original.lines;
+                        current_state.original_column = mapping.original.columns;
+                    }
+                }
+
                 b.appendMappingWithoutRemapping(current_state);
             }