Requirement and formatting fixes (#5)

Add ext-json to composer requirements
Update editorconfig to match core
Update phpdoc to not reference missing import

php-packages/testing/.editorconfig

@@ -15,5 +15,5 @@ indent_size = 2
 [*.{diff,md}]
 trim_trailing_whitespace = false
 
-[*.{php,xml}]
+[*.{php,xml,json}]
 indent_size = 4