Make cold bun install use 2x less memory (#3271)

* Make cold `bun install` use 2x less memory

In this benchmark: https://github.com/orogene/orogene/blob/main/BENCHMARKS.md

This brings us from around 2.7 GB to 1.2 GB of memory

* Address comments

---------

Co-authored-by: Jarred Sumner <709451+Jarred-Sumner@users.noreply.github.com>

src/string_mutable.zig

@@ -235,6 +235,13 @@ pub const MutableString = struct {
         return self.list.items;
     }
 
+    /// Clear the existing value without freeing the memory or shrinking the capacity.
+    pub fn move(self: *MutableString) []u8 {
+        const out = self.list.items;
+        self.list = .{};
+        return out;
+    }
+
     pub fn toOwnedSentinelLeaky(self: *MutableString) [:0]u8 {
         if (self.list.items.len > 0 and self.list.items[self.list.items.len - 1] != 0) {
             self.list.append(