Covers part of #5129 by adding `MoveToStartOfExcerpt`,
`MoveToEndOfExcerpt`, `SelectToStartOfExcerpt`, and
`SelectToEndOfExcerpt`.

No default linux bindings yet as it's unclear what to use. Currently,
`ctrl-up` / `ctrl-down` scroll up and down by one line (see #13269).
Considering changing the meaning of those.

Mac:

* Previously `cmd-up` and `cmd-down` were `editor::MoveToBeginning` and
`editor::MoveToEnd`. In singleton editors these will behave the same as
before. In multibuffers, they will now step through excerpts instead of
jumping to the beginning / end of the multibuffer.

* `cmd-home` and `cmd-end`, often typed as `cmd-fn-left` and
`cmd-fn-right` are now `editor::MoveToBeginning` and
`editor::MoveToEnd`. This is useful in multibuffers.

Release Notes:

- Mac: `cmd-up` now moves to the previous
multibuffer excerpt start, and `cmd-down` moves to the next multibuffer
excerpt end. Within normal buffers these behave the same as before, moving
to the beginning or end.