feat(completion): adds man page search, completion

[nicoandmee]

Description

I am a big fan of this project and humbly request this PR for consideration.

It adds support for man page completion by way of mankier. This site is very useful for reading command documentation in the browser. Here is an example of such a man page.

• This was relatively straightforward to accomplish by way of extending the completion BaseEngine.

Small refactor to use Fetch instead of XMLHttpRequest. I did this for two reasons:

• The MV3 spec requires that the background script use Fetch, as SW has no access to XMLHttpRequest.
• XMLHttpRequest has no equivalent of mode - without which API calls to the ManKier endpoint will fail, due to a CSP violation.

Thank you!

Please review the "Which pull requests get merged?" section in CONTRIBUTING.md.
✅

[nicoandmee]

@philc 👀

[nicoandmee]

@philc Any interest in getting this merged in the new year?

[aamehedi]

👍