I'm used to having the Emacs mark-sexp command, which selects/extends
the selection to the start or end of a sexp/larger syntax node, so when
I found out the `move to {start,end} of larger syntax node` commands
were added, these seemed like a natural addition; especially since most
other commands in Zed seem to have movement and selection pairs, but
those didn't really feel like they did.

I did a really *lot* of tests (brainstormed ideas with GLM 4.7, then
wrote the actual test case strings myself, then had it convert them to
Rust, then made sure the tests made sense and passed), I'm not sure if
it's too much or too little, but I looked at the PR that added the `move
to` versions of these commands and it was also +~550 lines, so this
seemed in the ballpark 😅

I factored out the core syntax node finding logic from those commands
into a common function, so that both sets of commands could use it, just
with different code for modifying the editor state wrapped around — mine
just moves each selection's head, instead of totally resetting it.

Release Notes:

- Added commands for extending selections to syntax node boundaries.
i.e. `editor: select to start of larger syntax node` and `editor: select
to end of larger syntax node`.