Add method host.createCustomSourceFile for custom file extensions

Suggestion

Hello, I like the new option --allowArbitraryExtensions in 5 TS, but this only works for created files on the file system or overriding a set of fs methods (readDirectory , readFile , etc).

Generating files on real time is not very efficient, so we have to override different methods so that the TS receives the created instance SourceFile without creating a file on filesystem.

I propose to add a new method createCustomSourceFile to the host, which is undefined by default, if the user has defined this method in the host, then the TS should call it for all files with extensions that the TS does not support (.css, .svg, .vue, etc).

📃 Motivating Example

host.createCustomSourceFile = function (fileName, sourceText): SourceFile {
  switch (fileName.slice(fileName.lastIndexOf('.'))) {
    case '.css':
      return ts.createSourceFile(fileName, getDtsOfCss(sourceText));

    case '.svg':
      return ts.createSourceFile(fileName, getDtsOfSvg(sourceText));
  }
};

💻 Use Cases

1. Bundle systems that always take care of importing files that TS doesn't support.
2. Custom language servers.

The issue names no repository file or test; start by tracing the host file-loading path and the existing fs overrides used with --allowArbitraryExtensions. Compare how unsupported extensions are handled, then define the host API behavior and tests for an optional createCustomSourceFile callback that supplies SourceFile instances without filesystem creation.