vim: Add support for moving to first, middle and last visible lines (H, L, M) (#6919)

This change implements the vim
[motion](https://github.com/vim/vim/blob/master/runtime/doc/motion.txt)
commands to move the cursor to the top, middle and bottom of the visible
view. This feature is requested in
https://github.com/zed-industries/zed/issues/4941.

This change takes inspiration from
[crates/vim/src/normal/scroll.rs](https://github.com/zed-industries/zed/blob/main/crates/vim/src/normal/scroll.rs).

A note on the behavior of these commands: Because
`NeovimBackedTestContext` requires compatibility with nvim, the current
implementation causes slightly non-standard behavior: it causes the
editor to scroll a few lines. The standard behavior causes no scrolling.
It is easy enough to account for the margin by adding
`VERTICAL_SCROLL_MARGIN`. However, doing so will cause test failures due
to the disparity between nvim and zed states. Perhaps
`NeovimBackedTestContext` should have a switch to be more tolerant for
such cases.

Release Notes:

- Added support for moving to top, middle and bottom of the screen in
vim mode (`H`, `M`, and `L`)
([#4941](https://github.com/zed-industries/zed/issues/4941)).

crates/editor/src/scroll.rs

@@ -299,7 +299,7 @@ impl ScrollManager {
 }
 
 impl Editor {
-    pub fn vertical_scroll_margin(&mut self) -> usize {
+    pub fn vertical_scroll_margin(&self) -> usize {
         self.scroll_manager.vertical_scroll_margin as usize
     }