Fix compiler warning when highlighting codeblocks in editor help

[Riteo]

It's more verbose but it should do the trick.

---

For context, it fixes the following error:

In file included from editor/scu/scu_editor.gen.cpp:24:
./editor/editor_help.cpp: In function 'void _add_text_to_rt(const String&, RichTextLabel*, Control*, const String&)':
./editor/editor_help.cpp:2627:32: error: this condition has identical branches [-Werror=duplicated-branches]
 2627 |                         } else if (lang == "csharp") {
      |                                ^~
cc1plus: all warnings being treated as errors
scons: *** [editor/scu/scu_editor.gen.linuxbsd.editor.dev.x86_64.o] Error 1
scons: building terminated because of errors.

(the fact that it's a SCU build doesn't matter AFAIA)

It happens because, without mono, the following:

			if (lang.is_empty() || lang == "gdscript") {
#ifdef MODULE_GDSCRIPT_ENABLED
				EditorHelpHighlighter::get_singleton()->highlight(p_rt, EditorHelpHighlighter::LANGUAGE_GDSCRIPT, codeblock_text, is_native);
#else
				p_rt->add_text(codeblock_text);
#endif
			} else if (lang == "csharp") {
#ifdef MODULE_MONO_ENABLED
				EditorHelpHighlighter::get_singleton()->highlight(p_rt, EditorHelpHighlighter::LANGUAGE_CSHARP, codeblock_text, is_native);
#else
				p_rt->add_text(codeblock_text);
#endif
			} else {
				p_rt->add_text(codeblock_text);
			}

gets preprocessed into this:

			if (lang.is_empty() || lang == "gdscript") {
				EditorHelpHighlighter::get_singleton()->highlight(p_rt, EditorHelpHighlighter::LANGUAGE_GDSCRIPT, codeblock_text, is_native);
			} else if (lang == "csharp") {
				p_rt->add_text(codeblock_text);
			} else {
				p_rt->add_text(codeblock_text);
			}

(this would also happen when disabling gdscript or both really)

[akien-mga]

Thanks!