Enable 'curly' rule (#1015)

Multi-line `if`/`for` statements in JS can be confusing since there
aren’t braces to indicate which code is enclosed in the statement. I set
the configuration to `multi-line` to enforce usage of braces for
multi-line statement bodies, but still allow things like `if (foo)
return;`. I additionally added the `consistent` option to require braces
for all elements of an if/else chain if one element has it. As you can
see, this set of options pretty closely matches the existing code style.

I was going to comment in #1008 about this stylistic change but realized
that it’s (IMO) a little impolite to ask for code style changes unless
they can be automatically enforced.

Note that `if (foo) { \n return; \n }` is still valid and won’t be
collapsed. I tried to automatically collapse all such cases but it was a
lot of files and I didn’t want to pick out the useful from the useless
differences.