Allow configurable indent size rather than enforced

[alpheusmtx]

Currently, Gleam enforces a 2-space indentation for all code. However, I think this is hard to read (for me) as I always use 4-space indentation in my projects. I believe that the Gleam formatter would be better by supporting different configuration, rather than enforcing a fixed style. Ideally, this could be set via the gleam.toml file or passed as a flag to the CLI, such as --indent or --indent-size.

[Goldenlion5648]

Agreed, this is one of the main blockers for me using gleam. It's hard to tell at a glance which block some code is in when there are only 2 spaces.

[Goldenlion5648]

I made a fork that changes the indent size to 4 spaces rather than 2.