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>

crates/editor/src/scroll/scroll_amount.rs

@@ -5,6 +5,8 @@ use ui::{Pixels, px};
 pub enum ScrollDirection {
     Upwards,
     Downwards,
+    Rightwards,
+    Leftwards,
 }
 
 impl ScrollDirection {
@@ -19,6 +21,8 @@ pub enum ScrollAmount {
     Line(f32),
     // Scroll N pages (positive is towards the end of the document)
     Page(f32),
+    // Scroll N columns (positive is towards the right of the document)
+    Column(f32),
 }
 
 impl ScrollAmount {
@@ -32,6 +36,15 @@ impl ScrollAmount {
                 }
                 (visible_line_count * count).trunc()
             }
+            Self::Column(_count) => 0.0,
+        }
+    }
+
+    pub fn columns(&self) -> f32 {
+        match self {
+            Self::Line(_count) => 0.0,
+            Self::Page(_count) => 0.0,
+            Self::Column(count) => *count,
         }
     }
 
@@ -39,6 +52,12 @@ impl ScrollAmount {
         match self {
             ScrollAmount::Line(x) => px(line_height.0 * x),
             ScrollAmount::Page(x) => px(height.0 * x),
+            // This function seems to only be leveraged by the popover that is
+            // displayed by the editor when, for example, viewing a function's
+            // documentation. Right now that only supports vertical scrolling,
+            // so I'm leaving this at 0.0 for now to try and make it clear that
+            // this should not have an impact on that?
+            ScrollAmount::Column(_) => px(0.0),
         }
     }
 
@@ -53,6 +72,8 @@ impl ScrollAmount {
         match self {
             Self::Line(amount) if amount.is_sign_positive() => ScrollDirection::Downwards,
             Self::Page(amount) if amount.is_sign_positive() => ScrollDirection::Downwards,
+            Self::Column(amount) if amount.is_sign_positive() => ScrollDirection::Rightwards,
+            Self::Column(amount) if amount.is_sign_negative() => ScrollDirection::Leftwards,
             _ => ScrollDirection::Upwards,
         }
     }