Declaration

[mhanberg]

Description

Port this feature from ElixirLS

https://github.com/elixir-lsp/elixir-ls/blob/master/apps/language_server/lib/language_server/providers/declaration/locator.ex

[sophiathedev]

I have a focused implementation ready that adds textDocument/declaration support for behaviour callbacks and protocol functions, including local and dependency-backed declarations, with Engine and provider regression tests.