Support empty export names in wasm2js and JS mangling in general

[kripken]

We were missing this silly validation rule, which explains the non-crash in #2288

[jgravelle-google]

Huh, I did not know that.

[kripken]

Actually this PR was totally wrong - empty exports are valid in wasm. They are just empty strings in JS, which is a valid identifier on the exports object... the spec and impls support that.

The proper fix here is then to just properly mangle such a name into something valid in JS, which I did as $.

[tlively]

I feel like if a user is crazy enough to have an empty string import then they might also be crazy enough to have a dollar sign import. Should there be some sort of error or fallback if "$" is already present?

[kripken]

There is a runtime error for the user when they try to run the code ;) I didn't see a nice way to add an error here - asmangle has to do something for such a string, and to detect the possible problem we'd need to introduce another layer of checks on top of all the callers to it.