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)).

assets/keymaps/vim.json

@@ -38,6 +38,8 @@
       "^": "vim::FirstNonWhitespace",
       "shift-g": "vim::EndOfDocument",
       "w": "vim::NextWordStart",
+      "{": "vim::StartOfParagraph",
+      "}": "vim::EndOfParagraph",
       "shift-w": [
         "vim::NextWordStart",
         {