Happy New Years! This PR is a second take at
https://github.com/zed-industries/zed/pull/38127 (cc @ConradIrwin)

This PR is significantly less complicated than the last attempt: while
we still keep our data on the `NavigationHistory` object, we no longer
tightly integrate it with the existing back/forward "browser history."
Instead, we keep our own stack of `(origin, target)` pairs (in a struct
to make it easy to extend with e.g., tag names in the future).

The PR is split into two separable commits. Most of the implementation
is in the second commit, which:
- Defines the stack data structure
- Implements `pane::GoToOlderTag` and `pane::GoToNewerTag` in terms of
the stack
- Hooks into `navigate_to_hover_links` to push tag stack entries

This last bit is the most fiddly. The core challenge is that we need to
keep track of the `origin` location and calculate the `target` location
across three codepaths that might involve creating a new editor and/or
splitting the pane. One thing in particular I found difficult was that
an editor's `nav_history` (an `ItemNavHistory`) seems to be populated
asynchronously. Instead of relying on it, I decided in this code to make
my own `ItemNavHistory`. I briefly tried to refactor the code in
question, but it seemed like it would significantly increase the scope
of the change.

I prefer this all-in-one tracking centered around
`navigate_to_hover_links ` to the `start/finish` approach taken in
https://github.com/zed-industries/zed/commit/b69a2ea200561de9de0edf4d0c4428ee887fb918
because I find it easier to convince myself that the right data is being
populated at the right times. Of course, let me know if you think
there's a better solution.

Closes #14206

Release Notes:
- ??? I don't know what to write here! Suggestions welcome