add yaml actionlint support (github actions)

[bretello]

Adds support for github action linter actionlint

[hsanson]

Please add entries for the new linter to ale-supported-languages-and-tools.txt and supported-tools.md. The list of tools must be listed in alphabetic order.

[hsanson]

Sorry, you also need to add the new linter entry to ale.txt section 7.6 in the same order as in the ale-supported-languages-and-tools.txt

[bretello]

Thanks! I wasn't sure why CI was failing. It should go through now.

Just as a note for whoever stumbling upon this, this is how I use this (to avoid having this lint all yaml files):

au BufRead,BufNewFile */.github/*/*.y.?ml let b:ale_linters = {'yaml': ['actionlint'] }

[hsanson]

Thanks, looking good.