message: Add `undefined` to TextDocuments#get(uri: string)
  As the documentation of this funtion states:
  > * Returns the document for the given URI. Returns undefined if
  > * the document is not mananged by this instance.
  The return type does not reflect that it can return undefined. This commit adds that.
diff: @@ -207,7 +207,7 @@ export class TextDocuments {
  * @param uri The text document's URI to retrieve.
  * @return the text document or `undefined`.
  */
  - public get(uri: string): TextDocument {
  + public get(uri: string): TextDocument | undefined {
  return this._documents[uri];
  }
  