jsg::Lock cleanup

[jclee]

Addresses a couple cleanup items on PR #547:

• Renames a few cases of jsgLock variables to js to match convention.
• Adds a jsg::Lock::v8Context() convenience function and updates code to use it.

[jclee]

A few things I wasn't quite sure about:

• having a member function v8Context() is convenient when used analogously to v8Isolate, but it differs from other workerd code in that it's a method that doesn't start with a verb. I think the convention would be getContext() or getV8Context(), but that's a bit more verbose. We also have a fair number of preexisting getContext() methods; not sure if having another might lead to confusion.

• I notice Worker::Lock has an analogous function -- another getContext() -- but it doesn't do quite the same thing, so I didn't try to rewrite code to use it, and likewise didn't rewrite things like lock.getIsolate()->GetCurrentContext().

• I generally left alone anything that wasn't obviously already using jsg::Lock, so there are a bunch of other places in the code still calling GetCurrentContext(). Mostly, they seem to be places where a v8::Isolate* parameter is being passed instead of a lock reference.

• Some places in the code seem to be jsg::Lock derivatives (e.g. JsgWorkerdIsolate::Lock), although I noticed they were using lock instead of js. Wasn't sure if this was intentional, so also I left them alone.