Add CodeCheck and CodeCheckVisitor for visitor based checks, adapt test bases
Migrate ForbiddenFunctionsCheck to CodeCheckVisitor
Migrate ForbiddenPHPFunctionsCheck to CodeCheckVisitor
Migrate VIPRestrictedClassesCheck to CodeCheckVisitor
Migrate VIPRestrictedCommandsCheck to CodeCheckVisitor, and update tests accordingly
Migrate CustomizerTest to CodeCheckTestBase
Have CustomizerTest cover use of WP_Customize_Image_Control and WP_Customize_Control classes
Migrate CustomizerCheck to CodeCheckVisitor
Migrate ForbiddenConstantsCheck to CodeCheckVisitor
Add test for ForbiddenConstantsCheck