This PR extends Lean's syntax to allow explicit universe levels in
expressions such as `e.f.{u,v}`, `(f e).g.{u}`, and `e |>.f.{u,v} x y
z`. It fixes a bug where universe levels would be attributed to the
wrong expression; for example `x.f.{u}` would be interpreted as
`x.{u}.f`. It also changes the syntax of top-level declarations to not
allow space between the identifier and the universe level list, and it
fixes a bug in the `checkWsBefore` parser where it would not detect
whitespace across `optional` parsers.
Closes#8743
