proposal: add microsoft/TypeScript issue list item to PULL_REQUEST_TEMPLATE

[nektro]

i think it would make sense to have an as optional item below mdn in the list. thoughts?
(posted the issue here since it would affect all spec repos if accepted)

[annevk]

Thus far we haven't done much collaboration with TypeScript, I think? I'd be more inclined to support this if the TypeScript community was more engaged on the various WHATWG standards and WPT development.