move `@oxlint/migrate` npm package to `oxlint`

Background

@oxlint/migrate is a tool, which converts ESLint flat configuration to an .oxlintrc.json file.
This tool was created when there was no integration between rust and js.

Because oxlint requires node, we could probably detect some command/flag like oxlint migrate or oxlint --migrate and execute the migration script.
Maybe we can even migrate the complete code into rust.

How the script works

It will search for a eslint.config.js file (or similar):
https://github.com/oxc-project/oxlint-migrate/blob/main/bin/config-loader.ts
Execute it:
https://github.com/oxc-project/oxlint-migrate/blob/124a25362f825961c163e8d6dd009617738e6b99/bin/oxlint-migrate.ts#L89-L91
and remaps the object structure to a compatible .oxlintrc.json file.
https://github.com/oxc-project/oxlint-migrate/blob/main/src/index.ts

Why?

1. Better integration with the supported rules
2. Maybe faster (because of rust)
3. No need for users to install another NPM package

Start by reviewing the linked @oxlint/migrate entry points: bin/config-loader.ts, bin/oxlint-migrate.ts, and src/index.ts, then compare how oxlint currently exposes commands or flags. Done means the migration workflow is integrated with oxlint, converts an ESLint flat configuration into .oxlintrc.json, and no longer requires users to install a separate npm package.