vim: Fix relative line motion

Before this change up and down were in display co-ordinates, after this
change they are in fold coordinates (which matches the vim behaviour).

To make this work without causing usabliity problems, a bunch of extra
keyboard shortcuts now work:

- vim: `z {o,c}` to open,close a fold
- vim: `z f` to fold current visual selection
- vim: `g {j,k,up,down}` to move up/down a display line
- vim: `g {0,^,$,home,end}` to get to start/end of a display line

Fixes: zed-industries/community#1562

crates/editor/src/display_map.rs

@@ -30,6 +30,7 @@ pub use block_map::{
     BlockDisposition, BlockId, BlockProperties, BlockStyle, RenderBlock, TransformBlock,
 };
 
+pub use self::fold_map::FoldPoint;
 pub use self::inlay_map::{Inlay, InlayOffset, InlayPoint};
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
@@ -310,7 +311,7 @@ impl DisplayMap {
 
 pub struct DisplaySnapshot {
     pub buffer_snapshot: MultiBufferSnapshot,
-    fold_snapshot: fold_map::FoldSnapshot,
+    pub fold_snapshot: fold_map::FoldSnapshot,
     inlay_snapshot: inlay_map::InlaySnapshot,
     tab_snapshot: tab_map::TabSnapshot,
     wrap_snapshot: wrap_map::WrapSnapshot,
@@ -438,6 +439,20 @@ impl DisplaySnapshot {
         fold_point.to_inlay_point(&self.fold_snapshot)
     }
 
+    pub fn display_point_to_fold_point(&self, point: DisplayPoint, bias: Bias) -> FoldPoint {
+        let block_point = point.0;
+        let wrap_point = self.block_snapshot.to_wrap_point(block_point);
+        let tab_point = self.wrap_snapshot.to_tab_point(wrap_point);
+        self.tab_snapshot.to_fold_point(tab_point, bias).0
+    }
+
+    pub fn fold_point_to_display_point(&self, fold_point: FoldPoint) -> DisplayPoint {
+        let tab_point = self.tab_snapshot.to_tab_point(fold_point);
+        let wrap_point = self.wrap_snapshot.tab_point_to_wrap_point(tab_point);
+        let block_point = self.block_snapshot.to_block_point(wrap_point);
+        DisplayPoint(block_point)
+    }
+
     pub fn max_point(&self) -> DisplayPoint {
         DisplayPoint(self.block_snapshot.max_point())
     }