Create Vim 9 script variant of the new indentation algorithm

[axvr]

Note
Currently waiting on #31 to be completed and merged.

Create a Vim 9 script variant of the new indentation algorithm to get significant performance improves during indentation.

This will be a fairly quick task, the only difficult part is figuring out how to integrate it in a clean way.

(Provide a secret config option to turn off Vim9 script version for testing.)

Neovim equivalent: #32