`using` and `await using` declarations (ES2026)

[ota-meshi]

Explicit Resource Management has now reached Stage 4. I would like Acorn to support the new syntax.

https://github.com/tc39/proposal-explicit-resource-management
tc39/ecma262#3000
https://arai-a.github.io/ecma262-compare/?pr=3000

Although it hasn't been written yet in tc39/proposals finished-proposals.md that it has reached Stage 4, the champion tweeted that it has reached Stage 4.
https://x.com/rbuckton/status/1927744228576129247