windows: fall back to `edit`, not to a notepad window

`C-a e` on a Windows with no editor installed opened notepad, which is
a GUI program: the pane it was spawned in drew nothing, so the split
sat there empty while the config opened in a window somewhere else.
The toast said as much, but a pane you have to be told to ignore is
still the wrong answer.

Windows 11 24H2 ships Microsoft Edit as `edit`, and it is a console
program -- it fills the pane like nvim or nano would, so the fallback
now has the same shape as the thing it is falling back from, and a
first-time user gets a usable editor without installing one.

It goes last of the console editors rather than first. Having fetched
nvim is a statement of preference; having been shipped `edit` says
nothing about what you want, so anything you chose yourself outranks
it. notepad stays behind it: `edit` is not on Windows 10, or on 11
before 24H2, and something that opens is better than nothing when the
alternative is an empty pane.

sl_which() already tries .exe, so it is found as installed; the search
still only runs when neither EDITOR nor the config asked for anything.

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