`prepareContent` should generate source maps

Is your feature request related to a problem? Please describe.

prepareContent is used to strip indentation and prepend some data to any file. Would be a nice addition to generate a source map of the changes so we can have errors with correct line/columns.

Describe the solution you'd like

Possibly use magic-string, which is already a svelte dependency to generate the map.

Describe alternatives you've considered

🤷

How important is this feature to you? Would improve the DX for the rest of the language-tools.