Support importing values/constants through `extern`

Use Case

Right now extern can be used to bring functions from the TypeScript/JavaScript, but it would be handy to be able to bring other kinds of values like arrays, strings, objects satisfying interfaces, etc.

It's possible to work around these limitations today by defining an extern function that returns the value you'd want, but it's an unnecessary restriction.

Proposed Solution

This will require some kind of alternative syntax for extern, e.g.

Syntax idea 1

bring extern "./foo.ts" {
  myValue: str;
  myOtherValue: Array<num>;
  myObj: IFoo;
};

log(myValue);
myObj.insert("region", "us-east-1");

Syntax idea 2

const foo = extern "./foo.ts" {
  myValue: str;
  myOtherValue: Array<num>;
  myObj: IFoo;
};

log(foo.myOtherValue);
foo.myObj.insert("region", "us-east-1");

Syntax idea 3

const foo = @extern("./foo.ts", {
  myValue: str;
  myOtherValue: Array<num>;
  myObj: IFoo;
});

log(foo.myOtherValue);
foo.myObj.insert("region", "us-east-1");

I think the syntaxes above need some work. An ideal syntax would support both use cases where the extern file exports an individual value and where it exports a complex object with multiple fields as shown above.

Implementation Notes

No response

Component

Language Design

Community Notes

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The issue names no implementation files, tests, or entry points. Start by reviewing the three proposed extern syntaxes and the Language Design component; the work is done only after a syntax is agreed that supports imported individual values and complex objects.