Add line length limit which includes indentation

[CobaltCause]

Description

The input is 81 columns wide. If I use --width 80, the output is the same as the input. If I incrementally decrease the width by one character, I find the output does eventually get reflowed at --width 76.

I experienced this on 187985f.

Small example input

    sprinkles = if sprinkles == null then import source.sprinkles else sprinkles;

Expected output

I personally prefer this, but I would also accept the output given with --width 76 which is shown later.

    sprinkles = if sprinkles == null
      then import source.sprinkles
      else sprinkles;

Actual output

At 80 columns:

    sprinkles = if sprinkles == null then import source.sprinkles else sprinkles;

At 76 columns:

    sprinkles =
      if sprinkles == null then import source.sprinkles else sprinkles;

[piegamesde]

Nixfmt currently does not include the preceding whitespace in the maximum line length calculation. That is intended behavior, it was once planned to add another flag to configure this but if never got implemented.