feat: add native JSON5 parser (Bun.JSON5) (#26439)

## Summary

- Adds `Bun.JSON5.parse()` and `Bun.JSON5.stringify()` as built-in APIs
- Adds `.json5` file support in the module resolver and bundler
- Parser uses a scanner/parser split architecture with a labeled switch
pattern (like the YAML parser) — the scanner produces typed tokens, the
parser never touches source bytes directly
- 430+ tests covering the official JSON5 test suite, escape sequences,
numbers, comments, whitespace (including all Unicode whitespace types),
unquoted/reserved-word keys, unicode identifiers, deeply nested
structures, garbage input, error messages, and stringify behavior

<img width="659" height="610" alt="Screenshot 2026-01-25 at 12 19 57 AM"
src="https://github.com/user-attachments/assets/e300125a-f197-4cad-90ed-e867b6232a01"
/>

## Test plan

- [x] `bun bd test test/js/bun/json5/json5.test.ts` — 317 tests
- [x] `bun bd test test/js/bun/json5/json5-test-suite.test.ts` — 113
tests from the official JSON5 test suite
- [x] `bun bd test test/js/bun/resolve/json5/json5.test.js` — .json5
module resolution

closes #3175

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

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

src/bun.js/ModuleLoader.zig

@@ -100,7 +100,7 @@ pub fn transpileSourceCode(
     const disable_transpilying = comptime flags.disableTranspiling();
 
     if (comptime disable_transpilying) {
-        if (!(loader.isJavaScriptLike() or loader == .toml or loader == .yaml or loader == .text or loader == .json or loader == .jsonc)) {
+        if (!(loader.isJavaScriptLike() or loader == .toml or loader == .yaml or loader == .json5 or loader == .text or loader == .json or loader == .jsonc)) {
             // Don't print "export default <file path>"
             return ResolvedSource{
                 .allocator = null,
@@ -112,7 +112,7 @@ pub fn transpileSourceCode(
     }
 
     switch (loader) {
-        .js, .jsx, .ts, .tsx, .json, .jsonc, .toml, .yaml, .text => {
+        .js, .jsx, .ts, .tsx, .json, .jsonc, .toml, .yaml, .json5, .text => {
             // Ensure that if there was an ASTMemoryAllocator in use, it's not used anymore.
             var ast_scope = js_ast.ASTMemoryAllocator.Scope{};
             ast_scope.enter();
@@ -361,7 +361,7 @@ pub fn transpileSourceCode(
                 };
             }
 
-            if (loader == .json or loader == .jsonc or loader == .toml or loader == .yaml) {
+            if (loader == .json or loader == .jsonc or loader == .toml or loader == .yaml or loader == .json5) {
                 if (parse_result.empty) {
                     return ResolvedSource{
                         .allocator = null,