fix: don't error if lspconfig is not installed

[mrcjkb]

I don't use lspconfig, but I currently use neoconf to tell neodev which plugins to load include in the lua-language-server's workspace.library (with lspconfig = false).

Since i removed lspconfig from my plugins, neoconf errors because it expects it to be installed.
This PR adds some checks to prevent that from happening.

I'm not sure if neoconf is useful without lspconfig (neodev is still adding the neovim lua API, but it doesn't seem to pick up the plugin lists in neoconf.json anymore).

[bugabinga]

I am in the same boat. Not using lspconfig, but would love to have neoconf overriding local lsp settings.

Can we reimplement the functions in util.lua, that depend on lspconfig to be independent?

[mrcjkb]

I am in the same boat. Not using lspconfig, but would love to have neoconf overriding local lsp settings.

Can we reimplement the functions in util.lua, that depend on lspconfig to be independent?

I have stopped using neoconf, so that would probably have to be done by someone else in a separate PR