Add support for `export defer`, when it lands

[wooorm]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything (or linked relevant results below)

Problem

https://tc39.es/proposal-deferred-reexports/#sec-exports

it’s being added to the AST over on estree/estree.

Current solutions

none

Proposed solutions

When it lands in estree and when it is in acorn and astring, add it here, and test everywhere.