Retrieve action version

**Describe the enhancement**
Method or an environment variable to retrieve action version specified in workflow.

**Code Snippet**
workflow.yml:
```yaml
steps:
 - uses: actions/my-action@v2
```

action code:
```js
const version = core.actionVersion();
// OR
const version = process.env.ACTION_VERSION
```

**Additional information**
This is useful when collecting usage statistics. 
