refactor(nvim): use explicit global `_G` table explicitly
refactor(nvim): use explicit global `_G` table explicitly

As recommended here:

    https://twitter.com/ahmedelgabri/status/1414310728396312580

`_G` is good because it is clearly an intentional assignment to the
global table. In contrast, this one:

    wincent = something

has two problems:

1. The Sumneko Lua LSP server is going to complain about a global that
   starts with a lowercase letter.
2. It might look like a mistake (a forgotten `local`).

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent