vim: Add horizontal scrolling support in vim mode (#32558)

Release Notes:

- Added initial support for both `z l` and `z h` in vim mode

These changes relate to #17219 but don't yet close the issue, as this
Pull Request is simply adding support for horizontal scrolling in vim
mode and actually moving the cursor to the correct column in the current
row will be handled in a different Pull Request.

Some notes on these changes:

- 2 new default keybindings added to vim's keymap
    - `z l` which triggers the new `vim::ColumnRight` action
    - `z h` which triggers the new `vim::ColumnLeft` action
- Introduced a new `ScrollAmount` variant, `ScrollAmount::Column(f32)`
to represent horizontal scrolling
- Replaced usage of `em_width` with `em_advance` to actually scroll by
the width of the cursor, instead of the width of the character

---------

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

assets/keymaps/vim.json

@@ -184,6 +184,8 @@
       "z f": "editor::FoldSelectedRanges",
       "z shift-m": "editor::FoldAll",
       "z shift-r": "editor::UnfoldAll",
+      "z l": "vim::ColumnRight",
+      "z h": "vim::ColumnLeft",
       "shift-z shift-q": ["pane::CloseActiveItem", { "save_intent": "skip" }],
       "shift-z shift-z": ["pane::CloseActiveItem", { "save_intent": "save_all" }],
       // Count support