Handle default exports

[jchavarri]

Fixes #697.

There is no good reason to export both $$default (as default is reserved word) and default, this leads to all sorts of issues with JS tooling.

The PR removes the former from the named outputs, allowing Melange users to export just the default value.

It is a breaking change, so maybe can be included in v2?

See both outputs in rollup repl, with both exports (notice the warning) and just the default.

For reference, this is the PR when the default handling was introduced, unfortunately there's not a lot of context or information about it: rescript-lang/rescript-compiler#2002.

[jchavarri]

Hm thinking more about it, the previous change would break any module that was defining a value with name default, which is probably not what we want.

In 6304e19, I changed the id that allows to identify default export to __es6__default_export, so there are no conflicts between both semantics. Happy to use different name or approach if this one could be problematic.

[anmonteiro]

I'm not so sure about this. It's quite the cognitive overhead to have to write let __es6__default_export = ....

[jchavarri]

we discussed we'll follow a different approach to fix this.