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.