Add editor actions for moving and selecting to next / previous excerpt

[mgsloan]

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