Allowing `using` declarations

[bakkot]

Per the readme/spec text, this only works for let or const. I think it should work for using as well. (And I guess await using also.) using specifically allows the RHS to be null or undefined, for cases where a resource needn't be acquired. Integrating with this proposal would let you do

if (using resource = maybeGetResource()) {
  // something with resource
} else {
  // handle the case of no resource
}

(Imagine, for example, a lockIfAvailable API which returns a disposable token to release the lock if the lock is available, or null if the lock is not available.)

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Having using work is also valuable for the "extended" form:

if (using resource = getResource(); resource?.needsUpdate()) {
  // something with resource
} else {
  // ideally the resource is disposed before this block executes
}

although the question of when the disposal happens depends on whether the binding is visible in the else, for which, see #3