qutebrowser: Add shellcheck search engine

Just run `sc <diagnosis number>` and you will instantly be beamed to the
correcsponding shellcheck wiki error page.

E.g. `sc 3001` will transport you to 'In POSIX sh, process substitution
is undefined'.

qutebrowser/config/searchengines.py

@@ -22,6 +22,7 @@ c.url.searchengines = {
     "pcw": "https://www.pcgamingwiki.com/w/index.php?search={}",
     "py": "https://pypi.org/search/?q={}",
     "r": "https://www.reddit.com/r/{}",
+    "sc": "https://www.shellcheck.net/wiki/SC{}",
     "sci": "https://sci-hub.ru/{}",
     "t": "https://www.thesaurus.com/browse/{}",
     "void": "https://voidlinux.org/packages/?q={}",