vim: Fix and improve horizontal scrolling (#33590)

This Pull Request introduces various changes to the editor's horizontal
scrolling, mostly focused on vim mode's horizontal scroll motions (`z
l`, `z h`, `z shift-l`, `z shift-h`). In order to make it easier to
review, the logical changes have been split into different sections.

## Cursor Position Update

Changes introduced on https://github.com/zed-industries/zed/pull/32558
added both `z l` and `z h` to vim mode but it only scrolled the editor's
content, without changing the cursor position. This doesn't reflect the
actual behavior of those motions in vim, so these two commits tackled
that, ensuring that the cursor position is updated, only when the cursor
is on the left or right edges of the editor:

-
ea3b866a76
-
805f41a913

## Horizontal Autoscroll Fix

After introducing the cursor position update to both `z l` and `z h` it
was noted that there was a bug with using `z l`, followed by `0` and
then `z l` again, as on the second use `z l` the cursor would not be
updated. This would only happen on the first line in the editor, and it
was concluded that it was because the
`editor:📜:autoscroll::Editor.autoscroll_horizontally` method was
directly updating the scroll manager's anchor offset, instead of using
the `editor:📜:Editor.set_scroll_position_internal` method, like
is being done by the vertical autoscroll
(`editor:📜:autoscroll::Editor.autoscroll_vertically`).

This wouldn't update the scroll manager's anchor, which would still
think it was at `(0, 1)` so the cursor position would not be updated.
The changes in [this
commit](3957f02e18)
updated the horizontal autoscrolling method to also leverage
`set_scroll_position_internal`.

## Visible Column Count & Page Width Scroll Amount

The changes in
d83652c3ae
add a `visible_column_count` field to `editor:📜:ScrollManager`
struct, which allowed the introduction of the `ScrollAmount::PageWidth`
enum.

With these changes, two new actions are introduced,
`vim::normal:📜:HalfPageRight` and
`vim::normal:📜:HalfPageLeft` (in
7f344304d5),
which move the editor half page to the right and half page to the left,
as well as the cursor position, which have also been mapped to `z
shift-l` and `z shift-h`, respectively.

Closes #17219 

Release Notes:

- Improved `z l` and `z h` to actually move the cursor position, similar
to vim's behavior
- Added `z shift-l` and `z shift-h` to scroll half of the page width's
to the right or to the left, respectively

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

assets/keymaps/vim.json

@@ -189,6 +189,8 @@
       "z shift-r": "editor::UnfoldAll",
       "z l": "vim::ColumnRight",
       "z h": "vim::ColumnLeft",
+      "z shift-l": "vim::HalfPageRight",
+      "z shift-h": "vim::HalfPageLeft",
       "shift-z shift-q": ["pane::CloseActiveItem", { "save_intent": "skip" }],
       "shift-z shift-z": ["pane::CloseActiveItem", { "save_intent": "save_all" }],
       // Count support