fix: prevent mason setup from being run twice (#1298) - #2

* fix: prevent mason setup from being run twice

Addresses #1297

Currently, we're calling `require('mason').setup(...)` twice:
* once when setting it as a dependency of `nvim-lspconfig` (since we set
	`config = true`)
* once in the `config` function we define for `nvim-lspconfig`

Calling setup twice can cause issues with, e.g., setting the `PATH`
option: you might append Mason's bin dir in one setup call and prepend
it in the other.

We've kept the setup of `mason` in the `nvim-lspconfig` dependencies
table since leaving it to the `config` function caused some
plugin-loading-order related issues in the past. See:
* #210
* #554
* #555
* #865

* docs: tweak comments per review feedback