GDScript: Add lambda syntax parsing

Lambda syntax is the same as a the function syntax (using the same
`func` keyword) except that the name is optional and it can be embedded
anywhere an expression is expected. E.g.:

    func _ready():
        var my_lambda = func(x):
            print(x)
        my_lambda.call("hello")

modules/gdscript/gdscript_tokenizer.cpp

@@ -242,6 +242,16 @@ void GDScriptTokenizer::set_multiline_mode(bool p_state) {
 	multiline_mode = p_state;
 }
 
+void GDScriptTokenizer::push_expression_indented_block() {
+	indent_stack_stack.push_back(indent_stack);
+}
+
+void GDScriptTokenizer::pop_expression_indented_block() {
+	ERR_FAIL_COND(indent_stack_stack.size() == 0);
+	indent_stack = indent_stack_stack.back()->get();
+	indent_stack_stack.pop_back();
+}
+
 int GDScriptTokenizer::get_cursor_line() const {
 	return cursor_line;
 }