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/search/src/buffer_search.rs

@@ -963,7 +963,7 @@ impl BufferSearchBar {
         done_rx
     }
 
-    fn update_match_index(&mut self, cx: &mut ViewContext<Self>) {
+    pub fn update_match_index(&mut self, cx: &mut ViewContext<Self>) {
         let new_index = self
             .active_searchable_item
             .as_ref()