[SAOC 2023] dfmt rewrite - Weekly update #6

[Prajwal S N]

Hi all,

Good news - the problem with using `appender!string` with the 
`extern (C++)` formatter class is no longer there. I've 
refactored the driver code to use a locking writer for both files 
and stdout. On a tangent, we've now found out that we cannot use 
`ASTBase` to build the AST - it does not store enough 
information, and also is missing some essential methods like 
`toChars()` that will inevitably be needed for the formatter to 
work. I've been working on using `ASTCodegen` instead, which 
comes with its own challenges of compiling imports. I hope to get 
it working and proceed with implementing a full traversal of the 
AST nodes soon.