[display] signature help for type parameters

It occured to me that we could trigger signature help not only for calls (on `(` and `,`), but also for type parameters (`<` and `,`). There's quite a bit of relevant info to be shown there:

- how many type parameters a function call / type expects
- what their names are
- what their constraints are, if any (ideally, expected type completion would work here)
- whether a type parameter is `@:const`
- whether a type parameter is a rest type parameter (I guess that follows from the name..?)