Automatic version detection

[uuf6429]

This pull request adds a new ruleset, PhpAuto, that picks the right ruleset based on the minimum required PHP version defined in composer.json of the current working directory.

New dependencies added:

1. ext-json (backword compatible, since it was already required by php-cs-fixer)
2. composer/semver (supported since PHP 5.3, so it should also be fine)

Additional Changes:

• project itself now uses the new ruleset (I figured it would be less maintenance + eating your own dog food)
• updated the readme file with the same reasoning; most people would want to automate this

Design Notes:

• at first I was going for a PHP version -> ruleset class (e.g. [53 => Php53::class]) map but then I thought it would be less work to just add an interface and build the class name dynamically

[uuf6429]

I've been thinking, this feature I'm proposing could also be implemented as a separate repo/package.

If that approach is preferred, I would gladly consent to having it under the same vendor.

[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 36 lines in your changes missing coverage. Please review.

Project coverage is 99.68%. Comparing base (b2b47fe) to head (62761f0).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/RuleSet/PhpAuto.php	0.00%	32 Missing ⚠️
src/PhpVersion.php	60.00%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##                main    #1098      +/-   ##
=============================================
- Coverage     100.00%   99.68%   -0.32%     
- Complexity        70       84      +14     
=============================================
  Files             22       23       +1     
  Lines          11310    11352      +42     
=============================================
+ Hits           11310    11316       +6     
- Misses             0       36      +36     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.