SourceMappingUrlProcessor (REGEX) does not apply to CSS source maps

[hleb-rubanau]

Processor is great but it only supports JS convention at the moment.

When I generate compressed CSS with dart-sass, sourcemapping URL comment looks like following (

/*# sourceMappingURL=application.css.map */

Unfortunately, its not taken into account by current preprocessor, which expects JS syntax (//# sourceMappingURL) both in parsing and in generation

Note #498