Allow the ability to format a DSL file

[rhamzeh]

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

[gurleensethi-docker]

Since we already have,

fga model lint

Something like,

fga model format -w .

would make sense.