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.

Das Issue nennt keine Repository-Datei und keinen Test; beginne damit, den Pfad nachzuverfolgen, über den der Host Dateien lädt, sowie die vorhandenen fs-Overrides, die mit --allowArbitraryExtensions verwendet werden. Vergleiche, wie nicht unterstützte Erweiterungen behandelt werden, und definiere anschließend das Verhalten der Host-API und die Tests für einen optionalen createCustomSourceFile-Callback, der SourceFile-Instanzen liefert, ohne Dateien im Dateisystem zu erstellen.