Use different delimiters for type args and variants

[osa1]

Currently we use brackets for both type arguments and variants, which cause nested brackets. From the tests in tests/Rows3.fir:

fn f2[r](): Result[[E1, E2, ..r], ()] = ...

fn f6[errs](f: Fn(): Result[[E1, ..errs], ()]): Result[[..errs], ()] = ...

Using angle brackets for one (probably type params, as that's more common so users will be familiar with it) makes sense, but we need to use a different syntax, or maybe just methods, for left and right shifts, to avoid the lexing and syntax highlighting issues with >> and <<s.

If we use angle brackets for type parameters:

fn f2<r>(): Result<[E1, E2, ..r], ()> = ...

fn f6<errs>(f: Fn(): Result<[E1, ..errs], ()>): Result<[..errs], ()> = ...

If we use angle brackets for variants:

fn f2[r](): Result[<E1, E2, ..r>, ()] = ...

fn f6[errs](f: Fn(): Result[<E1, ..errs>, ()]): Result[<..errs>, ()] = ...

Using braces is not an option, as we'll use braces for indentation insensitive syntax.