feat: add keyphrase-checker GitHub Action

[FidelusAleksander]

Changes

Added a new keyphrase-checker GitHub Action. This will help us with cases:

• checking a file was updated with expected content (whatever was tasked in the exercise)
• checking issue comment for keyphrases like @professortocat

example

    - name: Check for participant info in styles.css
      id: check-styles
      continue-on-error: true
      uses: skills/exercise-toolkit/actions/keyphrase-checker@minimum-occurances
      with:
        text-file: src/static/styles.css
        keyphrase: participant
        case-sensitive: false

Checklist

• I have added or updated appropriate labels to this PR
• I have tested my changes
• I have updated the documentation if needed

[FidelusAleksander]

Released in a separate repository
https://github.com/skills/action-keyphrase-checker