TS: Fix extraction of default-exported classes and functions

[asger-semmle]

Fixes #2051. I'm also running an evaluation just to be sure.

[ghost]

LGTM, but @erik-krogh should be the one to sign off on this.

[ghost]

(the change-notes are missing)

[erik-krogh]

LGTM. (But as Esben mentioned, change-notes are missing, I'll approve once those are in place)

However a comment.

Your fix is to change a ClassDeclaration to a ClassExpression in the case of a default export, because that is what the existing JavaScript extractor does.
However, as one can find out by using AST explorer, it is actually a ClassDeclaration.

So the real fix would be to change the JavaScript extractor to extract a ClassDeclaration, and then get that to work with all the existing code.

However, that is a fix for the backlog.
And I think this fix is good enough that it won't bite us later.

[asger-semmle]

However, as one can find out by using AST explorer, it is actually a ClassDeclaration.

That doesn't really matter though. Our AST was (initially) modelled after estree in which a ClassDeclaration must have an identifier. I checked the spec and AFAICT there is no semantic difference.

[asger-semmle]

Evaluation on typescript.slugs looks fine.