Supplying a source map URL doesn't make sense for a file importer, for
two reasons:

1. A canonical `file:` URL is already the recommended source map URL
   because it's always available on the local system.

2. Because a FileImporter doesn't know the fully-resolved canonical
   URL of the file it loads, it doesn't have any way of determining
   what the full source map URL should be anyway.

* main: (149 commits)
  Add the sourceMapIncludeSources option in the new JS API (#3226)
  Fix the toc tool to insert the toc (#3225)
  Fix some incorrect documentation/typings in LegacyPluginThis.options (#3246)
  Add a type annotation for the top-level NULL field (#3243)
  Add type annotations for the top-level TRUE and FALSE fields (#3241)
  Add type annotations for the sass.types.Error class (#3238)
  Add a type declaration for LegacyPluginThis.options.context (#3236)
  Make LegacyAsyncFunction type more usable (#3237)
  Allow SassFunction signatures to be checked on return (#3220)
  Make LegacyFileOptions.data optional (#3215)
  Use the URL type from the DOM definitions, not from Node (#3214)
  Document the new JS API (#3183)
  Make the new JS API a bit more idiomatic (#3200)
  Update immutable-js dep. (#3195)
  Add a code example for Logger.silent (#3192)
  Update dependencies (#3193)
  Replace FileImporterResult with a plain URL (#3180)
  Convert explicit compatiblity HTML into a nicer @-tag
  Convert explicit "Heads up" HTML into nicer-looking Markdown
  Automatically handle absolute file: URLs for FileImporters (#3181)
  ...