Improve CommonJS support

[ahejlsberg]

In this PR:

• Permit multiple module.exports assignments in a file and union the assigned types.
• Make JSDoc @typedef declarations available on imports of CJS modules that use module.exports.
• Exclude module.exports assignments from stricter checking of TypeScript export = declarations.

These changes align Corsa more closely with the current behavior in Strada and eliminate a large number of baseline differences.

[Copilot]

Pull request overview

This PR improves CommonJS support in the TypeScript compiler port by making three key changes:

1. Permitting multiple module.exports assignments and unioning their types
2. Making JSDoc @typedef declarations available on imports of CJS modules with module.exports
3. Excluding module.exports assignments from stricter TypeScript export = checking

These changes align Corsa more closely with Strada's behavior and eliminate numerous baseline differences.

Changes:

• Added mergedExportEquals field to ModuleSymbolLinks to track merged export types for CJS modules
• Updated test baselines to reflect removal of TS1203 errors for module.exports assignments
• Improved type resolution for CJS modules with multiple exports and typedefs

Reviewed changes

Copilot reviewed 295 out of 295 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
internal/checker/types.go	Added mergedExportEquals field to support merging multiple CJS exports
testdata/baselines/reference/submodule/**/*.diff	Removed TS1203 "Export assignment cannot be used when targeting ECMAScript modules" errors for module.exports
testdata/baselines/reference/submodule/**/*.types	Updated type information to show proper CJS module export types
testdata/baselines/reference/submodule/**/*.symbols	Updated symbol resolution for CJS exports
testdata/baselines/reference/conformance/**/*.errors.txt	Removed erroneous TS1203 errors from conformance tests