D lexer, parser, and ImportC parser are now standalone

[Walter Bright]

I've finally managed to sever its dependencies on the rest of the compiler. You 
can now build a standalone lexer, or a standalone D parser, or a standalone 
ImportC parser.

You can use it to build:

1. syntax highligher
2. source code formatter
3. syntax checker
4. your own semantic processor
5. something that annotates D code

Even without 1..5, having it be separate from the rest of the compiler makes it 
much easier to understand and work with.

https://github.com/dlang/dmd/pull/15091