[neovim] Disables Meson. Updates keymaps a bit.

home-manager/features/neovim/config/lua/plugins/disabled.lua

@@ -0,0 +1,4 @@
+return {
+    -- Maeson installs it's own binaries that are incompatible with NixOS.
+    { "williamboman/mason.nvim", enabled = false },
+}