Add `working-dir` for formatters

[vkcku]

Is your feature request related to a problem? Please describe.

I'm trying to use treefmt in a monorepo. Some commands are specific to certain projects and the commands are expected to be ran in the root directory of the project rather than the entire repo.

Describe the solution you'd like

An option working-dir per formatter that overrides the global working-dir option.

Describe alternatives you've considered

Right now, the only workaround is to wrap the formatters in a shell script to change the directory before running the command which is not the most pleasant experience.

Additional context

I think this was implemented in #53, but I cannot find such an option in the documentation now.

[brianmcgee]

I don't think this survived the move from Rust (v1) to Golang (v2). Should be straightforward enough to implement.