feat(dts): expose ts compiler errors

[danielroe]

It's possible for compiler errors to occur when processing individual declarations - this causes empty files which will (silently) break builds.

This exposes an errors object which can be handled by builders like unbuild to throw an error and fail the build in this case, if so configured. (It does not change default behaviour.)

While debugging, I removed the incorrect incremental compiler option and added the more permissive allowImportingTsExtensions.

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

src/utils/dts.ts:17

• [nitpick] The property name 'errors' is ambiguous. Consider renaming it to 'compilerErrors' to avoid confusion.

{ contents: string; errors?: Error[] }

src/utils/dts.ts:116

• The 'TypeError' constructor is being used incorrectly. The 'TypeError' constructor does not accept an options object with a 'cause' property. Consider using a custom error class or another approach to include the diagnostic information.

new TypeError(ts.formatDiagnostics([diagnostic], tsHost), { cause: diagnostic })

[pi0]

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