:gift: redraw cursor line at upper-middle by `z u`.

[t9md]

Introduce upper-middle with z space and z u. :bomb: Also rename confusing ScrollCursor{XXX} command name to RedrawCursorLineAt{XXX}

• z u conflicting pureVim's zuXXX spell-check related feature. But OK now, I have no plan to introduce these into vmp.

RedrawCursorLineAt{XXX} in viewport.
+-------------------------------------------+
| where        | no move | move to 1st char |
|--------------+---------+------------------|
| top          | z t     | z enter          |
| upper-middle | z u     | z space          |
| middle       | z z     | z .              |
| bottom       | z b     | z -              |
+-------------------------------------------+