Add methods to public API to determine the mode in use.

This is currently useful for migrating off ES5/3 while still using
already-cajoled content when possible.

It has been designed to be future-proof in case of the introduction
of a new or variant implementation strategy, by offering independent
answers to "is this ES5/3" and "is this SES" rather than an
enumeration (or boolean), which could not express e.g. "this is not
actually SES, but if you haven't heard of it you should treat it like
SES".

@ 99f9d8a81eff716ff93daa014fadd3493b0df0a0

[kpreid_google, 2015-04-27 20:38:35]

Please review for "is this a good idea" as well as "is the code good".

Also, this is the ES5/3-branch version; I'll also be putting a matching version
on trunk, er, master, without the conditional parts.

[MarkM, 2015-04-27 22:56:00]

LGTM

The general idea seems fine. I don't love it but I can't think of anything
better.