shellcheck: move common directives to .shellcheckrc

This simplifies our CI command line and makes it easier to document
expected usage (in HACKING.md).

There unfortunately isn't a way to set --checked-sourced or our default
warning level in .shellcheckrc, and our `find` command is still a bit
unwieldy, but this is still a net improvement.

.github/workflows/test.yml

@@ -978,8 +978,6 @@ jobs:
             --check-sourced \
             --color=always \
             --severity=warning \
-            --shell=bash \
-            --external-sources \
             $(find . \( -name "*.sh" -o -name "*.bash" \) -type f ! -path "./zig-out/*" ! -path "./macos/build/*" ! -path "./.git/*" | sort)
 
   translations: