AL64: Add linter for unapproved bashisms

Ash supports many bash specific extensions, but our codestyle only allows a few.

alpine/infra/docker/apkbuild-lint-tools!14 (merged) switches shellcheck in our CI to use the new busybox shell dialect, which shellcheck added in v0.10.0. This means it will no longer warn for the non-posix extensions that busybos as supports.

This linter will check for bashisms that are not approved:

• [[ test built-in
• == comparison in [
• &> and &>> redirections
• type -p and read -p
• source instead of .

Fixes #6 (closed)