Introduce strict mode to fail on warnings

Something like
```yml
- name: Analyze Terraform Plan
uses: byu-oit/github-action-tf-plan-analyzer@v2
with:
strict: true
```
or
```yml
with:
fail-on-warning: true
```

Start with the GitHub Action input configuration and trace how Terraform plan warnings determine the action result. Compare the proposed `strict` and `fail-on-warning` inputs, then verify that enabling the chosen input causes warnings to fail the action while the default behavior remains unchanged.

Written by the indexing model from the issue text.