Give a better message for multiple default exported functions of differing names

[DanielRosenwasser]

This is a follow up to @MartyIX's work on #5084.

With this change, instead of reporting Duplicate function implementation. for the following:

export default function foo() { }
export default function bar() { }

We will report A module cannot have multiple default exports.

However, for

export default function foo() { }
export default function foo() { }

we will report the original error message.

We also now correctly report on default-exported function declarations with no names.

[sandersn]

At this point we're not sure that it's a duplicate default export, right? Maybe the name should be different.
(I could be wrong, I've only skimmed areAllFunctionDeclarations... so far)

[DanielRosenwasser]

No, these are all declarations of the same symbol, so if one is a default export, they should all be default exports.

Still, that's a great assertion to check here. I'll add it.

[DanielRosenwasser]

Actually, I realized that we don't need the extra variable, we can just check the implementation.

[sandersn]

I don't know about our JSDoc style, but in the past I have skipped parameters that had no associated documentation. Is that allowed in JSDoc? Alternatively, instead of deleting this line, it would be nice to have some text here.

[DanielRosenwasser]

yeah I'll remove it