vim: { } to navigate by paragraph (#2668)

As part of this I added `assert_shared_state()` to the
NeovimBackedTestContext so that it is more like a drop-in replacement
for the VimTestContext.

The remaining part of zed-industries/community#682 is adding bracket
matching to plain text. It looks like the current logic requires there
to be a tree sitter language for the language in order to support
bracket matching. I didn't fix this in this PR because I was unsure
whether to try and work around that, or to try and add a plain text tree
sitter language.

Release Notes:

- vim: support `{` and `}` for paragraph motion
([#470](https://github.com/zed-industries/community/issues/470)).
- vim: fix `%` at the end of the line
([#682](https://github.com/zed-industries/community/issues/682)).

crates/vim/src/test/neovim_connection.rs

@@ -167,15 +167,25 @@ impl NeovimConnection {
             .await
             .expect("Could not get neovim window");
 
-        if !selection.is_empty() {
-            panic!("Setting neovim state with non empty selection not yet supported");
-        }
         let cursor = selection.start;
         nvim_window
             .set_cursor((cursor.row as i64 + 1, cursor.column as i64))
             .await
             .expect("Could not set nvim cursor position");
 
+        if !selection.is_empty() {
+            self.nvim
+                .input("v")
+                .await
+                .expect("could not enter visual mode");
+
+            let cursor = selection.end;
+            nvim_window
+                .set_cursor((cursor.row as i64 + 1, cursor.column as i64))
+                .await
+                .expect("Could not set nvim cursor position");
+        }
+
         if let Some(NeovimData::Get { mode, state }) = self.data.back() {
             if *mode == Some(Mode::Normal) && *state == marked_text {
                 return;