Add --php option to process command to run only PHP rules

[TomasVotruba]

Adds a --php option to the process command, that runs only PHP rules - rules bound to a minimal PHP version, i.e. rules implementing MinPhpVersionInterface.

Useful to split a PHP version upgrade from the rest of the rules, without having to maintain a second config:

vendor/bin/rector process src --php --dry-run

Example

With a config combining PHP sets and other sets:

return RectorConfig::configure()
    ->withPaths([__DIR__ . '/src'])
    ->withPhpSets(php84: true)
    ->withPreparedSets(deadCode: true, codeQuality: true);

Running vendor/bin/rector process src --dry-run applies everything:

 class Demo
 {
-    public function run(string $value = null)
+    public function run(?string $value = null)
     {
-        $unused = 1;
-
         return $value;
     }
 }

Running vendor/bin/rector process src --php --dry-run keeps only the PHP version rules:

 class Demo
 {
-    public function run(string $value = null)
+    public function run(?string $value = null)
     {
         $unused = 1;
 
         return $value;
     }
 }

Downgrade rules are not affected, as they do not implement MinPhpVersionInterface.

Like --only and --composer-based, the option narrows the run, so unused skip reporting is disabled to avoid false positives. It is forwarded to workers in parallel runs.