Feature Request: Allow UDLs in "Default language"

[pryrt]

While responding to "Save as .md default" on the forum, I found that there are two related feature requests implied in that discussion. I will submit those as two issues here, because they would affect two different sections of the codebase.

Please allow users to select from the available UDLs for Settings > Preferences > New Document > Default language.

This is a duplicate of the closed #979. In that conversation, @dail8859 claimed that it did work for some UDL. Unfortunately, after that claim, the OP never came back, and the conversation stagnated, so Meta closed out the issue four years later (but said if it was still an issue that a new issue could be created; hence, this new issue).

Dail had said that some UDLs show up in that list back in v6.8.3, but I don't remember that ever being true, and I haven't been able to replicate that on v8.2. (Also, the claim that the CoffeeScript UDL caused it to show up in v6.8.3 was possibly mistaken then, because I just downloaded [v6.8.3](http://download.notepad-plus-plus.org/repository/6.x/6.8.3/ and CoffeeScript was a builtin language in the Languages menu and the Settings pulldown, even back in 2015, so it was probably just the builtin CoffeeScript that he was seeing, not his CoffeeScript UDL.)

But since it's a dynamic list, it would be nice if when Notepad++ loads the existing UDL, it could add those as additional filetypes to the Default language pulldown. I understand if it's too many hooks to have it add every time a new UDL is defined or imported, but at least on program load, if it could add UDLs to the list, that would be great.

Notepad++ v8.2   (64-bit)
Build time : Dec 30 2021 - 03:40:42
Path : C:\usr\local\apps\npp\npp.8.2.portable.x64\notepad++.exe
Command Line : 
Admin mode : OFF
Local Conf mode : ON
Cloud Config : OFF
OS Name : Windows 10 Enterprise (64-bit) 
OS Version : 2009
OS Build : 19042.1466
Current ANSI codepage : 1252
Plugins : mimeTools.dll NppConverter.dll NppExport.dll 

[TFWol]

Googling around took me so long to end up at this Issue.

I would love to have this feature and am very surprised it hasn't been implemented yet.
There are many hardcoded things in npp that make no sense.