Suggestion: change code action for "underscoring" a variable to "rename to _var_name"

[megalithic]

Presently running lexical and next-ls side-by-side for a couple things and noticed lexical's code action for adding an underscore to a variable has a helpful detail, in that it explicitly calls out what it's doing and to which variable in the function head..

Could be a good first issue, since the pieces seem to all be there; that is, IF this is something the elixir-tools team might want/see valuable.

https://github.com/elixir-tools/next-ls/blob/main/lib/next_ls/extensions/elixir_extension/code_action/unused_variable.ex

[mhanberg]

seems like a fine change