Add .expert to Elixir.gitignore

Expert is the official language server implementation for the Elixir programming language.

Link: https://expert-lsp.org/

It stores its data in an `.expert` directory (similarly to Elixir LS). It is not meant to be tracked in the repository.

Expert doesn't have any proper documentation of this directory, but it's mentioned in FAQ: https://expert-lsp.org/docs/troubleshooting/

Author: mkaput

Elixir.gitignore

@@ -8,3 +8,4 @@ erl_crash.dump
 *.beam
 /config/*.secret.exs
 .elixir_ls/
+.expert/