refactor!: extract mason-lspconfig to separate plugin

[williamboman]

The rationale behind this is to make boundaries clearer as mason.nvim
has no direct relation with lspconfig per se.

Also, hopefully, by having it as a separate package like this would
encourage more people to write similar extensions (think mason-dap and
mason-null-ls). Ideally such extensions wouldn't be required at all, but
there are definitely gaps to fill as of today.

From now on you'll need to add williamboman/mason-lspconfig.nvim as
a plugin if you want to use the mason-lspconfig extension:

use {
    { "williamboman/mason.nvim", branch = "alpha" },
    "williamboman/mason-lspconfig.nvim",
    "neovim/nvim-lspconfig",
}

Plug "williamboman/mason.nvim", { 'branch': 'alpha' }
Plug "williamboman/mason-lspconfig.nvim"
Plug "neovim/nvim-lspconfig"