Vendor @hugoalh/http-header-link for CJS compat

The @hugoalh/http-header-link package only supports ESM, which causes
issues with CommonJS builds. This commit vendors the package by:

- Removing external dependency from deno.json and package.json
- Adding vendored implementation in src/runtime/link.ts
- Including comprehensive tests in src/runtime/link.test.ts
- Updating docloader.ts to use the vendored module
- Renaming HTTPHeaderLink to HttpHeaderLink for consistency

All existing functionality is preserved and tests pass.