feat: Support for Escoria and Game migrations (#473)

Co-authored-by: Dennis Ploeger <develop@dieploegers.de>

addons/escoria-core/_test/testversions/1.0.1.gd

@@ -0,0 +1,5 @@
+extends ESCMigration
+
+func migrate():
+	self._savegame.globals["test"] = "testb"
+

addons/escoria-core/_test/testversions/1.1.0.gd

@@ -0,0 +1,5 @@
+extends ESCMigration
+
+func migrate():
+	self._savegame.globals["test"] = "testc"
+