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

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---------

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

docs/guides/runtime/import-json5.mdx

@@ -0,0 +1,74 @@
+---
+title: Import a JSON5 file
+sidebarTitle: Import JSON5
+mode: center
+---
+
+Bun natively supports `.json5` imports.
+
+```json5 config.json5 icon="file-code"
+{
+  // Comments are allowed
+  database: {
+    host: "localhost",
+    port: 5432,
+    name: "myapp",
+  },
+
+  server: {
+    port: 3000,
+    timeout: 30,
+  },
+
+  features: {
+    auth: true,
+    rateLimit: true,
+  },
+}
+```
+
+---
+
+Import the file like any other source file.
+
+```ts config.ts icon="/icons/typescript.svg"
+import config from "./config.json5";
+
+config.database.host; // => "localhost"
+config.server.port; // => 3000
+config.features.auth; // => true
+```
+
+---
+
+You can also use named imports to destructure top-level properties:
+
+```ts config.ts icon="/icons/typescript.svg"
+import { database, server, features } from "./config.json5";
+
+console.log(database.name); // => "myapp"
+console.log(server.timeout); // => 30
+console.log(features.rateLimit); // => true
+```
+
+---
+
+For parsing JSON5 strings at runtime, use `Bun.JSON5.parse()`:
+
+```ts config.ts icon="/icons/typescript.svg"
+const data = JSON5.parse(`{
+  name: 'John Doe',
+  age: 30,
+  hobbies: [
+    'reading',
+    'coding',
+  ],
+}`);
+
+console.log(data.name); // => "John Doe"
+console.log(data.hobbies); // => ["reading", "coding"]
+```
+
+---
+
+See [Docs > API > JSON5](/runtime/json5) for complete documentation on JSON5 support in Bun.