Drop 'const' from 'using' declarations

[rbuckton]

This drops the const keyword from using declarations to reduce confusion over the fact BindingPattern is disallowed.

[github-actions]

A preview of this PR can be found at https://tc39.es/proposal-explicit-resource-management/pr/87.

[Jack-Works]

Suggested change

	using void = acquireFileHandle(); // block-scoped critical resource
	using acquireFileHandle(); // block-scoped critical resource

Is it possible?

[rbuckton]

Not really. `using` AssignmentExpression would conflict with using(x) (which is already legal JavaScript). The other point of using void = was that it could be used in a BindingList:

using x = ...,
      void = ...,
      y = ...;

[rbuckton]

`using` AssignmentExpression would conflict with using(x) (which is already legal JavaScript).

That is, unless we did something like `using` [lookahead != `(`] LeftHandSideExpression `;`, however I could see potential uses like the following that wouldn't work with the above:

using void = resource ?? getResource();

or

using void = (resource ??= getResource());