feat: allow pos="center", row=0, line=0

[l-kershaw]

Heavily based on the implementation from this PR by @jesseleite.

Also, added options to center in only one direction using 0 for row/line.
This required shifting the indices of everything else by one, which is a bit annoying, but this is how the implementation in vim works, so 🤷
This does break stuff in telescope, so I will write a PR over there when I get a chance.

[l-kershaw]

As mentioned here, the tests are currently broken as there is no longer a "nightly" tagged neovim release

[Conni2461]

LGTM if you are done we can update the notice date and merge this + the telescope PR

[Conni2461]

https://github.com/nvim-lua/plenary.nvim/pull/227/checks?check_run_id=3645392127#step:3:12

These stylua changes are probably thanks to the 0.11 release https://github.com/JohnnyMorganz/StyLua/releases/tag/v0.11.0 3 days ago.

[l-kershaw]

@Conni2461 thanks, I was getting very confused 😅
It's weird as its complaining about stuff that isn't even present in this PR

[jesseleite]

@l-kershaw Oh rad, so this does the same thing as in my PR here nvim-lua/popup.nvim#9!?

[l-kershaw]

@jesseleite yeh, thanks for writing that PR!
This was heavily based on your version, I just refactored it slightly so that we could also use the centering just horizontally or just vertically, using col=0 or row=0 respectively (as is done in vim).