Poll/discussion: dfmt option for single vs double indents for multi-line code

[Prajwal S N]

Hi everyone,

As a part of SAOC 2023, I've been working on porting dfmt to use 
the AST from DMD instead of libdparse (you might have come across 
my weekly updates). There are a few passes in the original dfmt 
that seem to allow for a purely aesthetic formatting 
inconsistency, and I wanted some community opinion on whether 
these should be made available in dmdfmt, or whether it would be 
better to retire them.

- `dfmt_single_template_constraint_indent`: If set to true, 
indents multi-line template constraints with a single tab. The 
default is false. E.g.
     ```d
     // default
     void foo(T)()
             if (is(T == char) || is(T == dchar))
     // true
     void foo(T)()
         if (is(T == char) || is(T == dchar))
     ```
- `dfmt_single_indent`: If set to true, indents multi-line 
function declarations with a single tab. The default is false. 
E.g.
     ```d
     // default
     void foo(int a, char b, string c, int d,
             char e, string f) {}
     // true
     void foo(int a, char b, string c, int d,
         char e, string f) {}
     ```

IMO, code on the following line should ideally be indented by a 
single tab, irrespective of the context of the newline. This 
ensures formatting consistency across all types of expressions. 
With these options, it causes the reader to flip between single 
tab and double tab indents across different items like 
if/else/for/while and function declarations/template constraints, 
which is quite jarring.

What do you think about this? Would it be better to maintain a 
single standard for indentation and not allow configuration, or 
should a knob be provided at the cost of consistency?