Add missing implementations to service host (#366)

Author: cevek

src/index.ts

@@ -278,6 +278,9 @@ export function register (options: Options = {}): Register {
       },
       getDirectories: getDirectories,
       directoryExists: directoryExists,
+      fileExists: fileExists,
+      readFile: getFile,
+      readDirectory: ts.sys.readDirectory,
       getNewLine: () => EOL,
       getCurrentDirectory: () => cwd,
       getCompilationSettings: () => config.options,