mojom-lsp doesn't support `result` response

[cfredric]

Mojo IDL now supports a result<T, E> response type: https://chromium.googlesource.com/chromium/src/+/main/mojo/public/tools/bindings/README.md#result-response

Example:

interface Foo {
  // A success would return an int32, whereas an error would return an E.
  MyMethod() => result<int32, Error>;
}

mojom-lsp does not support this syntax. It gives the following error (e.g.):

   --> 171:17
    |
171 |   MyMethod() => result<int32, Error>;
    |                 ^---
    |
    = expected '('