GDScript: add an optional message parameter to assert()

Before this patch, assert() only took the condition to assert on:

    assert(item_data)

Now, it can optionally take a string that will be printed upon failure:

    assert(item_data, item_name + " has no item data in ItemDatabase")

This makes it easier to immediately see what the issue is by being
able to write informative failure messages.

Thanks to @wiped1 for sharing their patch, upon which this is based.

Closes #17082

modules/gdscript/gdscript_compiler.cpp

@@ -1520,8 +1520,16 @@ Error GDScriptCompiler::_parse_block(CodeGen &codegen, const GDScriptParser::Blo
 				if (ret2 < 0)
 					return ERR_PARSE_ERROR;
 
+				int message_ret = 0;
+				if (as->message) {
+					message_ret = _parse_expression(codegen, as->message, p_stack_level + 1, false);
+					if (message_ret < 0)
+						return ERR_PARSE_ERROR;
+				}
+
 				codegen.opcodes.push_back(GDScriptFunction::OPCODE_ASSERT);
 				codegen.opcodes.push_back(ret2);
+				codegen.opcodes.push_back(message_ret);
 #endif
 			} break;
 			case GDScriptParser::Node::TYPE_BREAKPOINT: {