fix(parser): invalid main entrypoint in package.json

[AriPerkkio]

In #13432 the filename was changed but main remained as is. I'm now seeing following error in Vitest:

⎯⎯⎯⎯⎯⎯ Unhandled Error ⎯⎯⎯⎯⎯⎯⎯
Error: Failed to resolve entry for package "oxc-parser". The package may have incorrect main/module/exports specified in its package.json.
1  |  import { parse as BabelParser } from "@babel/parser";
2  |  import { parse as AcornParser } from "acorn";
3  |  import { parseSync as OxcParser } from "oxc-parser";
   |                                          ^
4  |  import { parseAst as ViteParser } from "vite";
5  |  import { expectTypeOf, test } from "vitest";
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

https://github.com/AriPerkkio/ast-v8-to-istanbul/actions/runs/17722503037/job/50357275013?pr=86

See errors from https://publint.dev/oxc-parser@0.88.0.

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[Copilot]

Pull Request Overview

This PR fixes an incorrect package entrypoint in the oxc-parser package.json file. The main entry was pointing to a non-existent index.js file when it should point to index.mjs, causing module resolution failures in consuming applications.

• Updates the main entrypoint from index.js to index.mjs to match the actual output file

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