feat: support github-actions-workflow language

[gadicc]

Resolves #1170.

• In extension.ts, adds { language: "github-actions-workflow } for the following schema: "file", "untitled", "deno". In each cases, added directly below the related entry for language: "yaml".

• In package.json, adds to the activationEvents array, "onLanguage:github-actions-workflow". THE SAME WAS ALSO ADDED FOR "onLanguage:yaml" which I assume was left out unintentionally.

Based on the same changes for vscode-yaml at https://github.com/redhat-developer/vscode-yaml/pull/1105.

Motivation: deno fmt already checks all yaml files (including github actions files) by default. If editing a deno project in vscode, we should be able to edit github actions files in vscode without them failing deno fmt --check in CI.