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.

El issue no menciona ningún archivo ni prueba del repositorio; empieza siguiendo el flujo de carga de archivos del host y los overrides de fs existentes que se usan con --allowArbitraryExtensions. Compara cómo se gestionan las extensiones no compatibles y, después, define el comportamiento de la API del host y las pruebas para un callback opcional createCustomSourceFile que proporcione instancias de SourceFile sin crear archivos en el sistema de archivos.