Format private named parameters

[munificent]

For Private Named Parameters, there is no real new syntax, so it likely doesn't require formatter changes. But technically this means allowing private identifiers where they aren't currently allowed. Just to be sure they do get formatted correctly, we should at least write some formatter tests.

[munificent]

The current parser already parses private named parameters and reports the error later in semantic analysis (which the formatter doesn't run), so at least for initializing formals, it's working already.

However, part of my implementation of private named parameters is to move some of that error checking into the parser (for invalid uses of them), so I'll also want to make sure the formatting still works after the formatter is using a version of analyzer with my changes and with the private-named-parameters experiment enabled.