GDScript: Show specific error when "yield" is used

To help people porting code, it gives a hint to use "await" instead of a
generic error.

modules/gdscript/gdscript_parser.cpp

@@ -2810,6 +2810,11 @@ GDScriptParser::ExpressionNode *GDScriptParser::parse_lambda(ExpressionNode *p_p
 	return lambda;
 }
 
+GDScriptParser::ExpressionNode *GDScriptParser::parse_yield(ExpressionNode *p_previous_operand, bool p_can_assign) {
+	push_error(R"("yield" was removed in Godot 4.0. Use "await" instead.)");
+	return nullptr;
+}
+
 GDScriptParser::ExpressionNode *GDScriptParser::parse_invalid_token(ExpressionNode *p_previous_operand, bool p_can_assign) {
 	// Just for better error messages.
 	GDScriptTokenizer::Token::Type invalid = previous.type;
@@ -3166,7 +3171,7 @@ GDScriptParser::ParseRule *GDScriptParser::get_rule(GDScriptTokenizer::Token::Ty
 		{ nullptr,                                          nullptr,                                        PREC_NONE }, // TRAIT,
 		{ nullptr,                                          nullptr,                                        PREC_NONE }, // VAR,
 		{ nullptr,                                          nullptr,                                        PREC_NONE }, // VOID,
-		{ nullptr,                                          nullptr,                                        PREC_NONE }, // YIELD,
+		{ &GDScriptParser::parse_yield,                     nullptr,                                        PREC_NONE }, // YIELD,
 		// Punctuation
 		{ &GDScriptParser::parse_array,                  	&GDScriptParser::parse_subscript,            	PREC_SUBSCRIPT }, // BRACKET_OPEN,
 		{ nullptr,                                          nullptr,                                        PREC_NONE }, // BRACKET_CLOSE,

modules/gdscript/gdscript_parser.h

@@ -1388,6 +1388,7 @@ private:
 	ExpressionNode *parse_attribute(ExpressionNode *p_previous_operand, bool p_can_assign);
 	ExpressionNode *parse_subscript(ExpressionNode *p_previous_operand, bool p_can_assign);
 	ExpressionNode *parse_lambda(ExpressionNode *p_previous_operand, bool p_can_assign);
+	ExpressionNode *parse_yield(ExpressionNode *p_previous_operand, bool p_can_assign);
 	ExpressionNode *parse_invalid_token(ExpressionNode *p_previous_operand, bool p_can_assign);
 	TypeNode *parse_type(bool p_allow_void = false);
 #ifdef TOOLS_ENABLED

modules/gdscript/tests/scripts/parser/errors/yield_instead_of_await.gd

@@ -0,0 +1,6 @@
+#GDTEST_PARSER_ERROR
+
+signal event
+
+func test():
+	yield("event")

modules/gdscript/tests/scripts/parser/errors/yield_instead_of_await.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+"yield" was removed in Godot 4.0. Use "await" instead.