Add an option to set the desired length for long lines

Currently, unless you set `split_long_lines:false` when using `Patdiff.Patdiff_core.patdiff` the line will split based on what is considered as the console length that is computed here

https://github.com/janestreet/patdiff/blob/836c423277c7c27f121e73f79b11737771040c8f/kernel/src/patdiff_core.ml#L419-L424

Couldn't this be the default value but customisable by adding an option `?max_line_length`?

Start with kernel/src/patdiff_core.ml at the linked lines 419-424 and inspect the Patdiff.Patdiff_core.patdiff interface. Confirm how the current console length is computed and make the requested maximum line length configurable while preserving the existing behavior by default.

Written by the indexing model from the issue text.