Equivalent to hitting cmd-d as many times as possible

cc: @JosephTLyons this PR needs a bit of work on user-facing naming and
interactions:
- [x] ~~I thought cmd-shift-d would be nice for this action, but that is
already taken by a sublime key binding. Could we use the VSCode binding?
I left the sublime text binding in but commented out.~~ Gonna just leave
it as is
- [x] ~~I went through 'SelectAllMatches' and 'SelectAll' as names for
this action, but ran into conflicts with the buffer search action and
the existing SelectAll (`cmd-a`) action. I decided to go with
`SelectNextAll`, but could use your help here.~~ Decided to go with
'SelectAllMatches'

Release Notes:
- Added a `editor::SelectAllMatches` command, bound to `cmd-shift-d`,
for selecting all matching occurrences under your selection. Note that
this has replaced the previous binding for `editor::DuplicateLine`.