extend insert mode with up/down/right/left and home/end key mappings

[itzmjauz]

This is similar to my last two PRs, I hope to extend basic movement in insert mode through arrow and home/end key mappings.
per : kakoune-keymappings;

This allows for basic movement while in insert mode and can occasionally prevent you from a couple keystrokes to change the cursor location.

I feel page-up and page-down could maybe also be added to insert mode. Though I'm not sure how useful that is.

[archseer]

I need to look but there was some issues with either history or the repeat operator (.) if the cursor moved from it's origin in insert mode.

[itzmjauz]

@archseer closing this for now, maybe this could be discussed in the keymap discussion.

[pickfire]

@itzmjauz If you want, try send small patches, movement in insert mode might affect repeat operator but ctrl-u might work maybe? I did add ctrl-w last time, not sure if it affects repeat operator.