initial draft of helmfile-linter

[dylanbannon]

Adds helmfile-linter action, which will print output from helmfile lint in the github actions console for every helmfile identified in a repo.

Tested using the cloudposse/helmfile-linter-test repo. Example of successful test and expected outputs: https://github.com/cloudposse/helmfile-linter-test/runs/1730194441?check_suite_focus=true.

NB: The cloudposse/helmfile-linter-test repo should be deleted once this PR is merged.

Design decisions:

• Helmfiles are identified in the repo according to this error message output by helmfile lint: It must be named helmfile.d/*.{yaml,yml}, helmfile.yaml, or charts.yaml, or otherwise specified with the --file flag, except that no effort is made to identify helmfiles with custom names (the ones that would need to be specified with the --file flag).
• Since the .github directory can contain a large number of yaml files, some of which could conceivably be named helmfile.yaml or charts.yaml, none of which are likely to be functional hemlfiles, helmfile-linter ignores any files in that directory.