Backend cleanup (#2859)

* Extender docblocks cleanup
* Excplicit type hinting in extenders
* Bring method under constructor
* Mark some classes and methods as internal
* Remove beta references

Co-authored-by: Clark Winkelmann <clark.winkelmann@gmail.com>

src/Frontend/Compiler/Source/StringSource.php

@@ -9,6 +9,9 @@
 
 namespace Flarum\Frontend\Compiler\Source;
 
+/**
+ * @internal
+ */
 class StringSource implements SourceInterface
 {
     /**