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

assets/keymaps/vim.json

@@ -137,10 +137,67 @@
           "partialWord": true
         }
       ],
+      "g j": [
+        "vim::Down",
+        {
+          "displayLines": true
+        }
+      ],
+      "g down": [
+        "vim::Down",
+        {
+          "displayLines": true
+        }
+      ],
+      "g k": [
+        "vim::Up",
+        {
+          "displayLines": true
+        }
+      ],
+      "g up": [
+        "vim::Up",
+        {
+          "displayLines": true
+        }
+      ],
+      "g $": [
+        "vim::EndOfLine",
+        {
+          "displayLines": true
+        }
+      ],
+      "g end": [
+        "vim::EndOfLine",
+        {
+          "displayLines": true
+        }
+      ],
+      "g 0": [
+        "vim::StartOfLine",
+        {
+          "displayLines": true
+        }
+      ],
+      "g home": [
+        "vim::StartOfLine",
+        {
+          "displayLines": true
+        }
+      ],
+      "g ^": [
+        "vim::FirstNonWhitespace",
+        {
+          "displayLines": true
+        }
+      ],
       // z commands
       "z t": "editor::ScrollCursorTop",
       "z z": "editor::ScrollCursorCenter",
       "z b": "editor::ScrollCursorBottom",
+      "z c": "editor::Fold",
+      "z o": "editor::UnfoldLines",
+      "z f": "editor::FoldSelectedRanges",
       // Count support
       "1": [
         "vim::Number",