Context-aware autocomplete and validation for action.yml runs section (#289)

- Set main as required in node-runs-strict schema definition
- Add validation for invalid key combinations based on using value
- Add validation for missing required keys (main for node, steps for composite, image for docker)
- Filter autocomplete suggestions based on using value
- Prioritize 'using' in completions when not set yet

Fixes context-aware autocomplete for action.yml files where different
action types (node, composite, docker) have different valid keys under runs:

workflow-parser/src/action-v1.0.json

@@ -267,6 +267,7 @@
           },
           "main": {
             "type": "non-empty-string",
+            "required": true,
             "description": "The file that contains your action code. The runtime specified in using executes this file.\n\n[Documentation](https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#runsmain)"
           },
           "pre": {