vim: Support gn command and remap gn to gl (#9982)

Release Notes:

- Resolves #4273

@algora-pbc /claim #4273

This is a work-in-progress. The process for `gn` command is:

- maintain updated vim.workspace_state.search.initial_query
- modify editor.select_next_state with
vim.workspace_state.search.initial_query
- use editor.select_next()
- merge selections
- set editor.select_next_state to previous state

To make this possible, several private members and editor structures are
made public. `gN` is not yet implemented and the cursor still does not
jump to the next selection in the first use.

Maybe there is an better way to do this?

---------

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

crates/workspace/src/searchable.rs

@@ -24,6 +24,15 @@ pub enum Direction {
     Next,
 }
 
+impl Direction {
+    pub fn opposite(&self) -> Self {
+        match self {
+            Direction::Prev => Direction::Next,
+            Direction::Next => Direction::Prev,
+        }
+    }
+}
+
 #[derive(Clone, Copy, Debug, Default)]
 pub struct SearchOptions {
     pub case: bool,