Make all filetype names lowercase

This may be a breaking change if you are using a plugin that
checks the filetype. All the default plugins are now updated,
just make the filetype you were checking for previously all
lowercase.

runtime/syntax/gdscript.micro

@@ -1,6 +1,6 @@
 ## GDScript syntax file, based on Python syntax file.
 ##
-syntax "GDScript" "\.gd$"
+syntax "gdscript" "\.gd$"
 
 ## built-in objects
 color constant "\b(null|self|true|false)\b"