GDScript: Consolidate behavior for assigning enum types

This makes sure that assigning values to enum-typed variables are
consistent. Same enum is always valid, different enum is always
invalid (without casting) and assigning `int` creates a warning
if there is no casting.

There are new test cases to ensure this behavior doesn't break in
the future.

modules/gdscript/tests/scripts/parser/warnings/enum_assign_int_without_casting.gd

@@ -0,0 +1,15 @@
+enum MyEnum { ENUM_VALUE_1, ENUM_VALUE_2 }
+
+# Assigning int value to enum-typed variable without explicit cast causes a warning.
+# While it is valid it may be a mistake in the assignment.
+var class_var: MyEnum = 0
+
+func test():
+	print(class_var)
+	class_var = 1
+	print(class_var)
+
+	var local_var: MyEnum = 0
+	print(local_var)
+	local_var = 1
+	print(local_var)

modules/gdscript/tests/scripts/parser/warnings/enum_assign_int_without_casting.out

@@ -0,0 +1,21 @@
+GDTEST_OK
+>> WARNING
+>> Line: 5
+>> INT_ASSIGNED_TO_ENUM
+>> Integer used when an enum value is expected. If this is intended cast the integer to the enum type.
+>> WARNING
+>> Line: 9
+>> INT_ASSIGNED_TO_ENUM
+>> Integer used when an enum value is expected. If this is intended cast the integer to the enum type.
+>> WARNING
+>> Line: 12
+>> INT_ASSIGNED_TO_ENUM
+>> Integer used when an enum value is expected. If this is intended cast the integer to the enum type.
+>> WARNING
+>> Line: 14
+>> INT_ASSIGNED_TO_ENUM
+>> Integer used when an enum value is expected. If this is intended cast the integer to the enum type.
+0
+1
+0
+1