Add GitHub Actions workflow to ensure new features are correctly named and documented #4493

Add workflow to ensure that PRs with titles like add xyz feature, include a file called xyz.tsx and add a line with xyz in readme.md. If they aren't found in the diff, it should fail.

This would have prevented this: https://github.com/sindresorhus/refined-github/issues/4483#issuecomment-863889718

And also fixes: https://github.com/sindresorhus/refined-github/issues/4320#issuecomment-833301366

Possible starting point, pseudo-code:

name: New feature checker

on:
  pull_request:
    types: [opened, edited]

jobs:
  checker:
    runs-on: ubuntu-latest
    if: contains(github.event.pr.title, 'Add') && contains(github.event.pr.title, 'feature')
    steps:
      - use: actions/checkout@v2
      - name: Find feature name
        id: feature 
        uses: AsasInnab/regex-action@v1
        with:
          regex_pattern: (?<=Add `)\w+(?=` feature)'
          search_string: github.event.pr.title
      - name: Has ${{ steps.feature.outputs.first_match }}.tsx been created?
        uses: mudlabs/simple-diff@v1.2.0
        with:
          path: source/features/${{ steps.feature.outputs.first_match }}.tsx 
      - name: Has the file been imported
        run: grep ${{ steps.feature.outputs.first_match }} source/refined-github.ts -q
      - name: Has the feature been documented
        run: grep ${{ steps.feature.outputs.first_match }} readme.md -q