[vim keymap] Add HML cursor motions

Here's the first draft of HML motions, I've also attached a demo in case someone would like to try out (the demo is not intended to be merged).

As usual, I'll need some comments from you guys.

I have some doubts on how shall I handle the half-occluded lines on top and bottom.
In original Vim, the scrolling of text is line-wise, but in CodeMirror, the scrolling is pixel-wise. At the current draft, I'm handling it by setting some torlerance/threshold to determine whether the line is visible enough to the user. It's just a naïve approach I could come up with.

Another thing to mention is that I've added `getUserVisibleLines()` to `vimApi` in order to allow users to reuse it from outer scopes. Do you think it's a good idea?