dub: JSON, SDL, YAML, TOML, what color should we paint it ?

[Mathias LANG]

Currently, Dub uses two formats: JSON and SDL.

JSON was there from the beginning, I assume due to Dub's npm 
inspiration.
In fact, we still very much depend on JSON internally: until 
recently, JSON object were passed around in quite a few places. 
When we download a package, regardless of the format the package 
recipe is in, we store it as `dub.json`.

But JSON is a terrible format to write configurations in, given 
how verbose it is, and it lacking support for comments.
So around 2014, we added support for SDL (no, not [this 
one](https://www.libsdl.org/), or [this 
one](https://web.fe.up.pt/~mricardo/05_06/amsr/artigos/sdlTutorial.pdf), but [this one](https://sdlang.org/), and it took me 10 minutes to find the link because Wikipedia doesn't even know about it).

At some point last year, I did some heavy refactoring on dub 
along with quite a few improvements. In the process of doing so, 
I took advantage of the fact that YAML is a subset of JSON to add 
[a YAML parser](https://github.com/dlang-community/D-YAML/), on 
which I have built a 
[library](https://github.com/dlang-community/configy) to do 
automatic configuration parsing, because let's be honest, it's 
2023 and we're using the language with the best compile-time 
reflection capabilities, so we really don't want to manually 
parse a standard file format to fill a struct.

This parser is used to parse all JSON files:
- `dub.json` (but not `dub.sdl`);
- `dub.selections.json`;
- `dub.settings.json`;

Now, I think that having `dub.settings.json` being written in 
JSON doesn't make sense, and I would like to [change 
this](https://github.com/dlang/dub/pull/2546). In fact, I have 
been wanting this for more than 3 years, and the people that 
[took part in the 
discussion](https://github.com/dlang/dub/issues/1832) broadly 
agreed.

Here are the options that have been floated around:
1) Use SDL everywhere;
2) Add support for YAML;
3) Add support for another format (E.g. TOML);

I would like to go with YAML, not because it is the greatest 
format on earth, but because SDLang is not well known (it means 
newcomer have to learn yet another format, and while not a big 
deal, it adds friction), while YAML is everywhere. In the [thread 
were SDL was 
discussed](https://forum.dlang.org/thread/gnfctbuhiemidetngrzi@forum.dlang.org), people even thought we invented it!

In practice, this would mean adding support for the file format 
with a YAML extension, giving it priority over the old format, 
then after a number of releases (most likely 10, probably more 
for `dub.json`), deprecating the JSON / SDL equivalent.

Obviously such a change would not happen overnight, and would 
need broad support from the community. Opinions ?