When a command used an explicit register (e.g. `"_dd` or `"add`),
the subsequent dot repeat (`.`) was ignoring that register and using
the default instead.

Store the register at recording start in `recording_register_for_dot`,
persist it to `recorded_register` when recording stops, and restore it
in `Vim::repeat` when no explicit register is supplied for `.`. An
explicit register on `.` (e.g. `"b.`) still takes precedence.

There were now 4 different places where we were setting the
`Vim::status_label` field and then calling `cx.notify`, so a helper
method, `Vim::set_status_label` was introduced, that simply receives the
label.

Since the notify call can be done on `Context<Editor>` was it was
already being done in `Vim::show_location`, we can drop the tracking of
the empty register and simply call the new method where we were updating
the `register_empty` variable to `true`.

Lastly, a new test that specifically checks how the repeat action deals
with registers has been introduced, and the existing test has been
refactored to use `assert_eq` instead of `assert_matches` so we get a
more visual representation of the expected output, making it easier to
update according to what we expect the result to be.

Rename `vim::state::VimGlobals::recorded_register` to
`vim::state::VimGlobals::recorded_register_for_dot` to make it more
explicit that this field is meant to be used for dot-repeating logic.