Implement user-configurable search completion providers

[rileywilddog]

Search completions are clearly useful enough to be a mainline feature, but the maintainer does not want to expand its usefulness by adding more providers. This PR allows users to benefit from the feature with more than the handful of built-in completion providers.

Adds the following tested examples:

rs: https://runescape.wiki/?search=%s @https://runescape.wiki/api.php?action=opensearch&format=json&search=%s Runescape Wiki
os: https://oldschool.runescape.wiki/?search=%s @https://oldschool.runescape.wiki/api.php?action=opensearch&format=json&search=%s Old School Runescape wiki
man: https://www.mankier.com/?q=%s @https://www.mankier.com/api/v2/suggest/?q=%s@results.*.text ManKier Man Pages

Fixes #4281, opened in case this PR is also rejected.
Mitigates the following closed or ignored PRs/issues to add engines:
#3851 #3981 #4091 #4094 #4189 #4238

[micangl]

@philc is there a chance for this PR to be merged before the next release? Currently, the only way to have custom search completion is by patching the source code.