Ability to customize used nix packages

[dundalek]

Resolves #15

Example how to use: https://github.com/dundalek/lazy-lsp.nvim/blob/aad718ab4155a5edb7963d3fd452bb2511d15bbf/notes.md#override-used-nix-packages

cc @zoriya

[zoriya]

Thank you for looking into this. I don't understand what

pcall(require("lspconfig").omnisharp.document_config.default_config.on_new_config, new_config, new_root_dir)

does and why the user should do it instead of lazy-lsp directly.

[dundalek]

Good question, it is kind of ugly, but it is to mimic the behavior of the on_new_config from lspconfig, when the user provides an override the original one is not called.

In the case of omnisharp the default function sets some parameters, so we call the original to make sure we get proper parameters before wrapping it with shell.

[zoriya]

Okay, I see, I completely forgot about that even thought I made the original PR for that.

Thank you for implementing that!