Add flags ala fnr

It would be really cool if you could press `` will searching to get a “flags area” to the left of the `/` in the command line like in [FNR](https://github.com/junegunn/vim-fnr). I suggest the following flags:
- `z`: Toggle fuzzy search.
- `r`: Toggle regex search (on by default) (this includes implementing a “literal” mode).
- `w`: Toggle word-boundary matching (off by default) (wraps the search in `\<` and `\>`).
- `v`: Toggle “very magic” mode (off by default) (prepends `\v`).

Mockup: `(zrwv) /search_term`.

No files or tests are named. Start by reading the existing `/` search command implementation and the FNR reference linked in the issue; done means the command line shows the proposed flags area and each listed toggle, including literal, boundary, and very-magic behavior, works as described.

Written by the indexing model from the issue text.