Make InitializeRequestArguments follow the protocol

[alt-romes]

It was previously making too many fields required and missing the supportsANSIStyling property

Fixes #17

[dmjio]

Nice find w/ supportsANSIStyling, and yes the nullability is off. Well done. Might be worth it to make smart constructors, with Nothing defaults, but we could probably do in a separate pass.

[alt-romes]

I don't think smart constructors would help library users that much, just add indirection.

Typically you're using the library to receive InitalizeRequestArguments in which case you probably do want to deal with the Maybe to set defaults. Using the library to write a client is also possible but much harder, as most RequestTypes don't instance ToJSON (only FromJSON), so it doesn't matter as much. In the debugger testsuite I now have a test which does construct a InitializeRequestArguments: https://github.com/well-typed/haskell-debugger/blob/f5b775764a78f2e3684c2c533357072983888f66/test/haskell/Test/DAP/RunInTerminal.hs

[dmjio]

Yea, that's a good point, we're receiving not constructing, maybe only for testing it would be relevant then.