feature: support range when entering zenmode.

[madelaney]

Did you check the docs?

• [X] I have read all the zen-mode.nvim docs

Is your feature request related to a problem? Please describe.

No, this is not currently related to a problem.

Describe the solution you'd like

I'd like to be able to select a range of lines, before enter zen-mode. Once I enter zen-mode, I'd like to only have the selected lines visible.

Describe alternatives you've considered

N/A

Additional context

I've tried a mix of folding, or smaller files, but there are some files that I just can't shrink (for work reasons) so being able to select a range before going into zen-mode seems more idiomatic of vim/neovim