fix(csv-generate): allow numeric and null column return types

[xiehuanyi]

ColumnsFunction requires a string result even though the generator supports strings, numbers and null. Match the declaration to those supported return values and regenerate the distributed declarations.

Convert the existing options.columns.js suite to TypeScript, retaining its string/null/number and invalid-return runtime coverage. This replaces the redundant added API test. The converted suite also exercises the existing Generator.end() method, so its missing end(): void declaration is included.

Validation: restoring the previous ColumnsFunction return type makes the existing converted column test fail to typecheck. npm test --workspace csv-generate passes typechecking and 79 tests (2 existing pending Web Stream tests); npm run lint:check --workspace csv-generate passes with the 2 existing pending-test warnings; npm run build --workspace csv-generate passes.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.

[wdavidw]

@xiehuanyi why did you close the PR ?