GDScript: Allow string keys on Lua-style dictionaries

Which is useful when the key isn't a valid identifier, such as keys with
spaces or numeric keys.

modules/gdscript/tests/scripts/analyzer/errors/dictionary_duplicate_key_lua.gd

@@ -0,0 +1,6 @@
+func test():
+	var lua_dict = {
+		a = 1,
+		b = 2,
+		a = 3, # Duplicate isn't allowed.
+	}

modules/gdscript/tests/scripts/analyzer/errors/dictionary_duplicate_key_lua.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Key "a" was already used in this dictionary (at line 3).

modules/gdscript/tests/scripts/analyzer/errors/dictionary_duplicate_key_lua_with_string.gd

@@ -0,0 +1,6 @@
+func test():
+	var lua_dict_with_string = {
+		a = 1,
+		b = 2,
+		"a" = 3, # Duplicate isn't allowed.
+	}

modules/gdscript/tests/scripts/analyzer/errors/dictionary_duplicate_key_lua_with_string.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Key "a" was already used in this dictionary (at line 3).

modules/gdscript/tests/scripts/analyzer/errors/dictionary_duplicate_key_python.gd

@@ -0,0 +1,6 @@
+func test():
+	var python_dict = {
+		"a": 1,
+		"b": 2,
+		"a": 3, # Duplicate isn't allowed.
+	}

modules/gdscript/tests/scripts/analyzer/errors/dictionary_duplicate_key_python.out

@@ -0,0 +1,2 @@
+GDTEST_ANALYZER_ERROR
+Key "a" was already used in this dictionary (at line 3).

modules/gdscript/tests/scripts/parser/features/dictionary_lua_style.gd

@@ -0,0 +1,9 @@
+func test():
+	var lua_dict = {
+		a = 1,
+		"b" = 2, # Using strings are allowed too.
+		"with spaces" = 3, # Especially useful when key has spaces...
+		"2" = 4, # ... or invalid identifiers.
+	}
+
+	print(lua_dict)

modules/gdscript/tests/scripts/parser/features/dictionary_lua_style.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+{2:4, a:1, b:2, with spaces:3}