Dynamic import syntax support

[guybedford]

I couldn't find an issue tracking this already, but let me know if it is a duplicate. This may well be implementable now that it is Stage 3 (estree/estree#137).

[ariya]

New productions:

CallExpression[Yield] :
  ImportCall[?Yield]

ImportCall[Yield] :
    import  (  AssignmentExpression[+In, ?Yield]  )

AST format addition:

interface Import {
    type: 'Import';
}

where it is being used:

interface CallExpression {
    type: 'CallExpression';
    callee: Expression | Import;
    arguments: ArgumentListElement[];
}

References:

• The proposal: https://tc39.github.io/proposal-dynamic-import/
• Change to the spec: ?
• ESTree change: Add experimental representation for dynamic import estree/estree#137

[3cp]

@ariya is it possible to release a minor version in v4 with this feature?