Add checkstyle output format

[TomasVotruba]

Closes #3033

Ref #2743

How to Add Rector to Code Reviews?

# see https://github.com/staabm/annotate-pull-request-from-checkstyle
# .github/workflows/rector_checkstyle.yaml

name: Rector Checkstyle

on:
    pull_request: null
    push:
        branches:
            - master

jobs:
    check_fixtures:
        runs-on: ubuntu-latest
        steps:
            -   uses: actions/checkout@v2
            -   uses: shivammathur/setup-php@v1
                with:
                    php-version: 7.2
                    coverage: none # disable xdebug, pcov
                    tools: cs2pr

            -   run: composer install --no-progress
            -   run: |
                    bin/rector process src --set dead-code --dry-run--output-format=checkstyle | cs2pr

Thanks @staabm for inspiration ❤️

[staabm]

Really cool !

[nicholasruunu]

@TomasVotruba Was this removed? Can't seem to find it.

[TomasVotruba]

Yes, see rectorphp/rector-src@f03cfa7

In the hindsight, this was not aligned with automated upgrade that Rector should be.
The checkstyle is suitable rather for static analyzers like PHPStan rules. If there is a need for checkstyle with Rector, you should make/look for a PHPStan rule.