Consider adding a native fuzzy path picker

This is a big feature request and is probably out of scope, though it is also a sort of thing which I think fits fish general philosophy of making reasonable things work well out of the box. Basically, I want fish to have `fzf --fish` built-in.

Here's a specific use-case. I am in directory `foo`, and I want to run a command passing a file from directory bar. My shell looks like this

```
matklad@TigerMac ~/p/matklad.github.io (master)
λ deno task spell ~/p/blog/
```

what I want to do is to "auto-complete" the path to a file somewhere _deep_ in the blog directory. The UX I want here is a fuzz file picker.

The good news here is that, due to how awesome UNIX philosophy is, I can achieve this by stringing together other tools. I just need to know that `fzf` solves the problem of fuzzy-finding, and `fd` solves the problem of listing files, so I can consult various manpages, chuck

```
set -gx FZF_CTRL_T_COMMAND 'fd --type f'
fzf --fish | source
```

to my fish.config, and then press `^T` for all the fzf glory.

The bad news is that, unless you do full-time UNIX, the philosophy isn't very user-friendly. It took me years to realize that "fuzzy picking a path argument" is a thing I might want to want, about an hour to figure out how to actually achieve that, and today I realized that my solution doesn't actually work, because, in the above example, my fzf thingy doesn't actually pick `~/p/blog` as a root directory to search into, and instead looks inside `~/p/matklad.github.io`. Given that I use this feature once in a blue moon, it just doesn't make sense for me to figure out how to improve that.

Contrast this with beloved fish auto-suggestion feature, which just works even before you know you need it.

I think it'd be great if, instead of tab, a fish user could press something like ctrl-tab, and get a command-aware path picker.

Given that fish is implanted in Rust these days, all individual components should be available off the shelf?