feat(navigation): add support for vi bindings (h = left and l = right)

[sassman]

This PR replaces #608 because it seems to be that the original author is gone.

Also, I propose to only handle the keys for rightArrow (l) and leftArrow (h) here, for the key bindings to navigate back and forth for whole sections in the sidebar I am planning to bring up a new PR.

[ehuss]

Thanks for the PR! I'm a little reluctant to merge something like this because the existing keybindings have caused various issues with extensions and input fields (see #1020, #1244). I'm uncertain, but maybe the keybindings should inspect the target similar to how the search bindings work? Unfortunately, I don't fully understand the issues involved, so I'm not sure if that makes sense.

[sassman]

@ehuss thanks for the feedback. I will have a closer look at those mentioned issues and see if your suggestion is applicable in that context.

[sassman]

PS: just keep in mind that on almost every site even here on GitHub there are key bindings available.
? for a help overlay, / for the search, j and k for navigation purpose. So I'd say that is some sort of best practice.

[szabgab]

I think I'd start by creating a help overlay to show the existing mappings.
Then I'd create a way for the author of the book to define mapping in the book.toml file and would default LeftArrow and RightArrow as it is now.

[rustbot]

☔ The latest upstream changes (possibly #2681) made this pull request unmergeable. Please resolve the merge conflicts.