Add experimental TS7 native API mode for transpile/emit workflows

[Copilot]

This introduces a scoped, opt-in native TypeScript 7 path in ts-loader focused on transpile/emit use cases, without attempting full parity with the existing compiler integration. The existing TypeScript path remains unchanged while native mode is added behind explicit guardrails.

• Scope and activation

  • Adds experimentalNativeApi loader option (default false).
  • Native mode is explicitly constrained to emit-focused usage (transpileOnly: true).

• Native adapter path (parallel, not compiler swap)

  • Adds a dedicated native integration layer (src/native.ts) instead of reusing legacy compiler assumptions.
  • Creates and reuses a native API session/snapshot lifecycle across rebuilds.

• Per-file emit + output mapping

  • Uses native per-file emit APIs for JS and source maps.
  • Maps native emit outputs into ts-loader’s expected { outputText, sourceMapText } contract.
  • Captures declaration outputs for webpack asset emission when declaration-related options are enabled.

• Diagnostics integration

  • Pulls native diagnostics (emit/program/global/config parsing), de-duplicates, and surfaces them through existing webpack error plumbing.
  • Respects existing loader-level diagnostic filtering behavior.

• Dependency/watch compatibility

  • Reworks dependency registration in native mode from native project/program metadata and config files.
  • Preserves webpack-facing dependency tracking behavior.

• Initial guardrails for unsupported features

  • Native mode currently rejects: getCustomTransformers, projectReferences, experimentalWatchApi, custom module/type resolvers, and happyPackMode.
  • Emits clear runtime option errors for unsupported combinations.

• Docs and compatibility fixtures

  • Documents the new opt-in mode and limitations in README.md.
  • Adds native-mode comparison/execution fixtures and updates related option-validation snapshots.

// webpack.config.js
{
  test: /\.ts$/,
  loader: 'ts-loader',
  options: {
    transpileOnly: true,
    experimentalNativeApi: true,
    compiler: 'typescript-native-preview' // npm alias to TS7 dev build
  }
}

[johnnyreilly]

Closing in favour of #1704 which seems more promising