fix(cli): use map file name in sourceMappingURL, not the full output path

[JamBalaya56562]

Summary

When --sourcemap is used with an output file in a subdirectory (e.g. lightningcss --bundle bundle.css -o dist/my-package.css --sourcemap), the emitted /*# sourceMappingURL=... */ comment contained the full output path instead of just the map file name:

# before:  /*# sourceMappingURL=dist/my-package.css.map */   ❌
# after:   /*# sourceMappingURL=my-package.css.map */         ✅

Because the URL is resolved relative to the CSS file's own location, the dist/ prefix makes the browser look for dist/dist/my-package.css.map, which breaks source maps for the common src -> dist project layout described in #794.

This changes the CLI to emit only the map file's name in the comment. The .map file itself is still written next to the output file using the full path, exactly as before.

Changes

• src/main.rs — derive the sourceMappingURL value from output_file.file_name() (falling back to the full <output>.map string if the path has no file-name component). The path used to write the .map file is unchanged.
• tests/cli_integration_tests.rs — update the existing sourcemap test (which previously asserted the full path) to expect the basename, and add sourcemap_url_relative_to_output_subdir, a regression test reproducing the exact subdirectory scenario from the issue.

Notes

• No behavior change when the output has no directory component: -o out.css still yields /*# sourceMappingURL=out.css.map */.
• Only the CLI emits this comment; the napi/Node binding returns the source map as a separate object and is unaffected.

Fixes #794

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