feat: Add K/J keys for Focus{Prev,Next}SameKind

[busfahrer]

(Disclaimer: I found out about this project because of jj-vcs)

I really like this tool for editing diffs, the only gripe I have with it that even though 90% of all commands are reachable using vim-style keys from the home row, one very common operation, namely jumping to the next/prev item at the same hierarchy level, forces me to leave home row.

This simple patch remedies this adding the analogous K and J key mappings for these operations.

edit: In case it's not clear, the keys are Shift+K and Shift+J

[pheki]

I just realized you implemented the exact same feature I had already implented in #113 haha

(which says something about the demand for this!)

[emesterhazy]

Thanks for this PR! I think #113 is closer to being merged since it already updates the menu items. Let's move forward with that version.