ProjectAnalyzer consumed Config::$fileExtensions early in its
constructor - without having processed plugins' modifications,
registering their custom scanners or analyzer implementations.
This change
* adds new specific interface \Psalm\Plugin\FileExtensionsInterface
to be used by plugin implementations
* extracts file extension handling from \Psalm\PluginRegistrationSocket
and interface \Psalm\Plugin\RegistrationInterface to a new dedicated
\Psalm\PluginFileExtensionsSocket and new interface
\Psalm\Plugin\FileExtensionsInterface
!!! this is a breaking change in PluginRegistrationSocket !!!
* adds runtime in-memory cache for Config::$plugins
* calls new method Config::processPluginFileExtensions(), providing
modifications to file extension only early in ProjectAnalyzer
* adjusts documentation
