Improve elixir-ts--thing-settings.

Improve elixir-ts--thing-settings.

* lisp/progmodes/elixir-ts-mode.el (elixir-ts--thing-settings):
Add extra grouping "keywords" to named sexp nodes.
Add "," to anonymous sexp nodes.  Use bos/eos.

* lisp/progmodes/ruby-ts-mode.el (ruby-ts-mode):
Use bos/eos for anonymous sexp nodes for better disambiguation.

* test/lisp/progmodes/heex-ts-mode-tests.el
(heex-ts-mode-test-indentation): Skip the test when 'elixir' is
missing since 'heex-ts-mode' depends on the 'elixir' grammar.

(cherry picked from commit cd557d6f6480cae81c39a451b2a2aaee720ba985)