Include undefined in template value types - #88

The bundled PrimitiveValue type excludes undefined, so strict TypeScript rejects optional fields and explicit undefined values that expand() already accepts. Include undefined in the alias to match the existing scalar, list and record handling.

The consumer fixture covers these inputs and keeps functions, symbols and deeper objects outside the accepted types. It produces six TS2322 errors with the original declaration and none with this change.

Fixes #79.

Checks:

• TypeScript 5.9.3: tsc --strict --module NodeNext --target ES2022 --noEmit type-tests/expand.ts
• npm test on Node 18.20.8: 159 tests pass, with no failures or skips, using the pinned URI-template test corpus.

The compiler fixture is checked separately from npm test; no compiler dependency or package script is added.