vim: Add support for vim::PreviousLineStart motion (#14193)

Release Notes:

- vim: Added `-`/`+` to go to beginning of line above/below
([#14183](https://github.com/zed-industries/zed/issues/14183)).
- vim: (Breaking) Removed non-standard builtin binding from `-` to open
the project panel. You can re-add it to your keymap file with:
`{"context":"VimControl", "bindings":{ "-":
"pane::RevealInProjectPanel"}}`


Optionally, include screenshots / media showcasing your addition that
can be included in the release notes.


https://github.com/zed-industries/zed/assets/32429059/0e9e9348-265e-4a81-a45a-4739034dc5d9

---------

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

assets/keymaps/vim.json

@@ -12,6 +12,8 @@
       "down": "vim::Down",
       "enter": "vim::NextLineStart",
       "ctrl-m": "vim::NextLineStart",
+      "+": "vim::NextLineStart",
+      "-": "vim::PreviousLineStart",
       "tab": "vim::Tab",
       "shift-tab": "vim::Tab",
       "k": "vim::Up",
@@ -192,7 +194,6 @@
       "ctrl-w g shift-d": "editor::GoToTypeDefinitionSplit",
       "ctrl-w space": "editor::OpenExcerptsSplit",
       "ctrl-w g space": "editor::OpenExcerptsSplit",
-      "-": "pane::RevealInProjectPanel",
       "ctrl-6": "pane::AlternateFile"
     }
   },