Support Custom GitHub Action use of GITHUB_ENV

**Is your feature request related to a problem? Please describe.**
Some GitHub Actions use `GITHUB_ENV` to set up custom environment variables.
When used inside a workflow, use `${{ env.CUSTOM_VARIABLE }}` is flagged as `Context access might be invalid`.

**Describe the solution you'd like**
Enable the custom GitHub Actions developer to define statically or dynamically the generated environment variables (like for the action output) and use it in this plugin to remove false-positive.

**Additional context**

This workflow is flagged with the issue

```yaml
name: "Context access might be invalid"
on: push
jobs:
context-access-might-be-invalid:
runs-on: ubuntu-latest
steps:
- name: Checkout
uses: actions/checkout@v3

- name: Setup environment variables
uses: rlespinasse/github-slug-action@v4

- name: Print custom environment variables
run: echo ${{ env.GITHUB_REF_SLUG }}
```
And produces

No source file, test, or entry point is named in the issue. Start by tracing how the extension validates env context and how custom action outputs are represented; compare that behavior with the GITHUB_ENV workflow example. Done means custom environment variables from actions can be declared or resolved without the false-positive diagnostic.

索引モデルが issue の本文から書いたものです。