DLS deprecation

[Laurent Tréguier]

I started working on this project to make it more comfortable to 
write D back in 2017, published a VSCode extension a couple 
months later, and continued working on it throughout 2018. In 
2019 however, I slowed down, and eventually, stopped working on 
it.

It was fun, and kept me well occupied for quite some time; but I 
have been working on something else since April of last year, and 
since I don't have any use for D in it, I am not taking time to 
do anything with DLS.

So today, I am deprecating DLS, along with its editor extensions. 
If anyone was using them, be advised that they will not have any 
update or support from now on.

Webfreak is still working on code-d/serve-d from what I gather, 
so hopefully, the handful of people who could be using DLS on 
VSCode can use it instead.