Handle .yml files the same as .yaml

Mostly this is so we can lint gqlgen.yml, but also other yaml config
files where the tooling decides how the file is named.

Auditors: benkraft

Test plan:
In webapp, ran with success:
   git ls-files '*.yml' | xargs ka-lint

runlint.py

@@ -419,6 +419,7 @@ def find_files_to_lint(files_and_directories,
                    '.jsx': 'javascript',
                    '.less': 'less',
                    '.yaml': 'yaml',
+                   '.yml': 'yaml',
                    '.kt': 'kotlin',
                    '.go': 'go',
                    '.graphql': 'sdl',   # graphql "schema definition language"