Add source map merging for preprocessors and get tests passing.
- fixed some issues around the `sources` array where they weren't calculated relative to the input correctly
- adjusted some CSS tests because due to our own CSS parser the AST is less granular, so there are less mappings now. Don't think this is a problem, but worth thinking about
- removed enableSourcemap but only log a warning, the reason this was introduced was to mitigate a bug in Vite which occured when having the source map inlined into the SSR'd output. Since SSR doesn't contain inlined CSS anymore (and if it did, we would omit the source map) the reason for which it was introduced no longer exists
- files without js mapping in it have no source mappings yet (originally added in Svelte 4 for #6092)
Simon H