fix: improve jumps and class qualifiers

[ObserverOfTime]

Upon reviewing nvim-treesitter/nvim-treesitter#5527 I noticed that (a) the jump expression labels are anonymous nodes and (b) the _this_at and _super_at rules are unused. This change makes it easier to capture the labels and classes and ensures that qualified calls are parsed as expected. I wanted to use token.immediate("@") in all of the *_at rules but the label rule would take precedence (even after editing it and raising the other rules' precedence) so I had to compromise.

[ObserverOfTime]

I suppose this fixes #5.

this@A.foo = null

(source_file [0, 0] - [1, 0]
  (assignment [0, 0] - [0, 17]
    (directly_assignable_expression [0, 0] - [0, 10]
      (this_expression [0, 0] - [0, 6]
        (type_identifier [0, 5] - [0, 6]))
      (navigation_suffix [0, 6] - [0, 10]
        (simple_identifier [0, 7] - [0, 10])))))

It also takes care of #43.

[ObserverOfTime]

Hello? :)

[VladimirMakaev]

@ObserverOfTime looked at your PR, seems legit and not super risky, merging.