Adds an `expert` server entry (binary `expert`, invoked `expert --stdio`)
to the built-in LSP defaults, auto-detected for Mix projects
(`mix.exs`/`mix.lock`). Placed after `elixirls` so elixir-ls remains the
primary navigation server when both binaries are installed; this keeps the
default getServerForFile() behavior unchanged. Absent-binary setups are
unaffected by loadConfig's PATH/root-marker gating.
Includes a docs row, changelog entry, and a regression test pinning that
`.ex` files match `expert` and that `elixirls` still sorts ahead of it.
can1357