feat: add equalprg

[heygarrett]

Closes #555

Vim and Neovim have built-in formatting functionality:

• :help equalprg
• :help C-indenting

This PR adds equalprg as a formatter which does the following:

1. Store current cursor position in a variable
2. gg (<range-start>G if range is provided)
3. =G (=<range-end>G if range is provided)
4. Restore cursor position from variable

[stevearc]

This may appear to work in the simple case, but it will cause errors as soon as you try to run it with more than one formatter. Lua function formatters need to

1. not modify the buffer
2. pass the formatted text back as an argument to the callback

[heygarrett]

Ah, I didn't realize the buffer can't be modified. Are there any existing formatters that use a Lua function but act directly on text in a buffer that I could look at as an example? I wonder if using a temporary file would work. 🤔

[stevearc]

There aren't any other formatters that do something like that right now. I agree that a temporary buffer is probably the way to go for this, but it'll likely be a bit tricky. Usually the FileType autocmd will set the indentexpr or equalprg on the buffer, but if you try to set the filetype on a temp buffer it won't work as expected during format_on_save or format_after_save because we're already in an autocmd. You'll likely need to manually copy over the relevant option values to the temp buffer.