Support ... in list patterns

[rodrigues]

... now works inside list patterns, so you can match lists by their edges and ignore the middle:

Pattern	Matches
[...]	any list, including empty
[1, ...]	any list whose leading element is 1
[x, y, z, ...]	any list of ≥3 elements, capturing the first three
[..., y, z]	any list of ≥2 elements, capturing the last two
[first, ..., last]	captures both ends; ... absorbs the middle

Fixed-length list patterns ([1, 2, 3], [_, _, _]) are unchanged — still an exact-length match.

Before

$ mix ex_ast.search "[first, ..., last]" router.ex

0 match(es)

After

$ mix ex_ast.search "[first, ..., last]" router.ex
router.ex:3
  [
    {:get, "/", Home},
    {:get, "/about", About},
    {:post, "/users", Users}
  ]
  first: {:get, "/", Home}
  last: {:post, "/users", Users}

1 match(es)

$ mix ex_ast.search "[{:count, _}, ...]" lib
lib/mix/tasks/ex_ast.search.ex:74
  [
    count: :boolean,
    limit: :integer,
    allow_broad: :boolean,
    format: :string,
    json: :boolean,
    expand_imports: :boolean
  ]

1 match(es)

Also collapses a duplicate-match quirk where every literal (lists, tuples, atoms, strings, numbers) was reported twice.