feat: Add a GitHub Actions workflow for use in CI workflows

[Lamparter]

This PR implements an alternative way for consuming Uno.Check - from GitHub Actions. This will appear in the GitHub Marketplace once a release on the repo is published or a tag is created.

This also removes the need for the massive unnecessary workflow files shipped with the Uno templates wizard downstream.

---

name: Example Workflow
on: [push]

jobs:
  build:
    runs-on: windows-latest
    steps:
      - name: Checkout code
        uses: actions/checkout@v4
      - name: Install Uno Platform workload
        uses: unoplatform/uno.check@[version]
        with:
          target-platform: 'all'
          dotnet-version: '8.0.x'
          sdk-version: '19041'

[CLAassistant]

All committers have signed the CLA.

[DevTKSS]

Hello @Lamparter @jeromelaban , I noticed this PR hasn’t moved since the last exchange, and I was wondering if there’s an update on how the Uno team plans to proceed. If this is still considered relevant, I’d be happy to help move it forward.