Run doctests in CI and fix up existing doctests (#37851)

Follows on from
https://github.com/zed-industries/zed/pull/37716#pullrequestreview-3195695110
by @SomeoneToIgnore

After this the doctests will be run in CI to check that the examples are
still accurate.

Note that doctests aren't run by Nextest: you can run them locally with
`cargo test --doc`.

Summary:
* Run tests from CI
* Loosen an exact float comparison to match approximately (otherwise it
fails)
* Fixed one actual bug in the tests for `dilate` where the test code
assumed that `dilate` mutates `self` rather than returning a new object
* Add some `must_use` on some functions that seemed at risk of similar
bugs, following the Rust stdlib style to add it where ignoring the
result is almost certainly a bug.
* Fix some cases where the doc examples seem to have gone out of date
with the code
* Add imports to doctests that need them
* Add some dev-dependencies to make the tests build
* Fix the `key_dispatch` module docstring, which was accidentally
attached to objects within that module
* Skip some doctest examples that seem like they need an async
environment or that just looked hard to get running

AI usage: I asked Claude to do some of the repetitive tests. I checked
the output and fixed up some things that seemed to not be in the right
spirit of the test, or too longwinded.

I think we could reasonably run the tests on only Linux to save CI
CPU-seconds and latency, but I haven't done that yet, partly because of
how it's implemented in the action.

Release Notes:

- N/A

.github/actions/run_tests/action.yml

@@ -21,3 +21,8 @@ runs:
     - name: Run tests
       shell: bash -euxo pipefail {0}
       run: cargo nextest run --workspace --no-fail-fast
+
+    - name: Run doctests
+      shell: bash -euxo pipefail {0}
+      # Nextest currently doesn't support doctests
+      run: cargo test --workspace --doc --no-fail-fast