This PR makes searching in vim mode significantly more like vim.

I re-used search to implement "go to next instance of word under cursor"
as this is how it works in vim (for integration with other
search-related keyboard shortcuts) and to avoid having to rewrite all
the logic to be vim-specific; but that did mean I had to make some
changes to the way search works (in particular to allow different
searches to run with specific options).

Release Notes:
- vim: `<enter>` in search now puts you back in normal mode
([#1583](https://github.com/zed-industries/community/issues/1583))
- vim: `?` now works to search backwards.
- vim: jumping to definitions or search results keeps you in normal mode
([#1284](https://github.com/zed-industries/community/issues/1284))
([#1514](https://github.com/zed-industries/community/issues/1514))
- vim: `n`/`N` are now supported to jump to next/previous match after a
search
([#1583](https://github.com/zed-industries/community/issues/1583))
- vim: `*`/`#`/`g*`/`g#` are now supported to jump to the next/previous
occurrence of the word under the cursor.
- vim: `gD` now jumps to type definition