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refactor(list): deprecate List::new / empty in favour of List([]) (#4289)

`List::new()` (with its `empty` alias and the `@list.new()` /
`@list.empty()` free functions) only returns the empty list. Unlike the
immutable maps and sets, where `new()` avoids the `Hash`/`Compare`
bound that `T([])` needs, `List(xs : ArrayView[A])` has no bound, so
`List([])` works everywhere `new()` does and follows the `T(xs)`
constructor convention. (`Empty` is not an option outside the package:
`List` is a read-only enum.)

Move `new` to deprecated.mbt, deprecate the method, the `empty` alias
and both free-function forms with "Use `List([])` instead", and migrate
in-repo callers.

Part of #4272.
Codex review: approved; follow-up applied (the `@list.default`
deprecation message now points to `List([])`).

Signed-off-by: Codex CLI <codex@openai.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

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