Closes #1776

There are 2 separate issues addressed here that both contribute to the issue:

1. In rollup config file we failed to include ts.Extension, resulting in ts.Extension become undefined in the bundle.

2. In js/compiler.ts we did not specify that we allow files of non-TS extensions => This would result in the following lines to fail:
   https://github.com/Microsoft/TypeScript/blob/ed8c81a5638c7d745cb8541137ad6990da045467/src/compiler/program.ts#L2151-L2161

function getSourceFileFromReferenceWorker(...) {
  // ...
  const sourceFileNoExtension = options.allowNonTsExtensions && getSourceFile(fileName);
  if (sourceFileNoExtension) return sourceFileNoExtension;
  // ...

  const sourceFileWithAddedExtension = forEach(supportedExtensions, extension => getSourceFile(fileName   extension)); // Trying with all possibly supported extensions
  // ...
}

Without allowNonTsExtensions set to true in options, the compiler would try out all possible file names with extensions but never check the original extensionless one. This causes the behavior in debug log as mentioned in #1776 (comment)

/cc @kitsonk