elixir-ts-mode: Bring the faces' use closer to other ts modes

elixir-ts-mode: Bring the faces' use closer to other ts modes

* lisp/progmodes/elixir-ts-mode.el (elixir-ts--font-lock-settings):
Rename feature 'elixir-function-name' to 'elixir-definition' and
update all deferences.  Add parameters' highlighting with
font-lock-variable-name-face.  Change variable references'
highlighting to use font-lock-variable-use-face.
Move the feature 'elixir-variable' from feature level 3 to level 4, to
match other ts modes (bug#67246).

(cherry picked from commit eb90fb52b08a16ae2bdc8bad6929492b9e693f72)