Allow the ability to format a DSL file

We would like to expose a new method: formatter.Format(dslFile, options).

Given a DSL file contents, it would return a formatted DSL file.

Options may not be much at the start, but can grow to include:

• using spaces instead of tabs
• tab width
• having a space between types
• grouping assignable relations at the top
• grouping non-assignable relations prefixed with is_ or can_ at the bottom

For modular models, we should allow passing a map containing the fga.mod and the contents of each file, but that should be a separate ticket.

In later tickets:

• this should be integrated into the VS Code ext, IntelliJ plugin and CLI
• We should allow formatting the store file

Also potentially later combined with a linter used in a github action

Start by reviewing the requested formatter.Format(dslFile, options) API and the proposed formatting options. The issue scopes initial work to formatting a single DSL file; modular models, editor and CLI integrations, store-file formatting, and linting are explicitly later work. Done means the agreed initial API formats DSL input according to its supported options.