[SAOC 2023] dfmt rewrite - Weekly update #1

[Prajwal S N]

Hi everyone,

For SAOC 2023, I'm working on refactoring 
[dfmt](https://github.com/dlang-community/dfmt) to use the AST 
from DMD-as-a-library instead of libdparse.

The past week has been very interesting. I got up to speed with 
the dfmt codebase, and managed to do a 1-to-1 port of the lexer 
dependency from libdparse to DMD-as-a-library. Most parts were 
pretty straightforward, and the bulk of the work was replacing 
every `tok!"<token>"` instance with `TOK.<token>` and making sure 
the token coming from DMD was the same as what was previously 
being used. So far so good!

You can see the draft PR tracking the work 
[here](https://github.com/dlang-community/dfmt/pull/589).

Going forward, my mentor and I have decided that it would be 
impractical to try and replace the parser directly, for multiple 
reasons:

- It's a lot of work to replace the parser and use the DMD AST 
instead of libdparse's, and all of this work will happen without 
a working version of dfmt. If, at the end of this, dfmt is broken 
or refuses to compile, it could very well mean that all that 
effort went down the drain.
- Doing a brute force replacement of the parser will prevent us 
from testing the transformation passes in dfmt individually, and 
also brings us back to the point above.

Hence, we've decided to do an incremental rewrite of the files 
that use the parser, initially with no passes (just to ensure the 
AST is being built in the first place), and then adding each pass 
along with relevant unit tests.