deps: enable Sorbet strict typing

Add Sorbet `typed: strict` sigil and full type signatures to
`cmd/deps.rb`, promoting the last non-strict brew command to
strict typing.

Changes:
- Add `sig` type signatures to all private methods
- Replace block parameter in `deps_for_dependents` with a
  `DepsCombineMode` enum for better type safety
- Initialize `@use_runtime_dependencies` with `T.let` in constructor

Ref: https://github.com/Homebrew/brew/issues/17297