[Extensions WebIDL] Add support for descriptive IDL promise comments

This CL adds a way to provide a name and description for the value a
promise return resolves to by using a comment similar to what we do for
function arguments. This is necessary to retain parity with how we used
to extract these values from the function arguments when they were
defined as function callbacks rather than promise based returns.

Also adds associated test.

Bug: 340297705
Change-Id: I79f89fcd0752ec5f612ebe252e17fb0266773fee
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6598899
Commit-Queue: Tim <tjudkins@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1470839}