Add support for static variables in GDScript

Which allows editable data associated with a particular class instead of
the instance. Scripts with static variables are kept in memory
indefinitely unless the `@static_unload` annotation is used or the
`static_unload()` method is called on the GDScript.

If the custom function `_static_init()` exists it will be called when
the class is loaded, after the static variables are set.

modules/gdscript/tests/scripts/runtime/features/static_variables.out

@@ -0,0 +1,16 @@
+GDTEST_OK
+Inner._static_init inner
+InnerInner._static_init inner inner
+data: data
+perm: 0
+prop: prefix Hello! suffix
+perm: 1
+prop: prefix World! suffix
+other.perm:0
+other.prop:prefix Hello! suffix
+other.perm:2
+other.prop:prefix foo suffix
+load.perm:0
+load.prop:prefix Hello! suffix
+load.perm:3
+load.prop:prefix bar suffix