Closes #4906 (comment).

Investigation

• The compiler exports any invalid identifier as $$<identifierName>. For example, let else = "hello" is valid ReScript, but not valid JS because else is a keyword. Therefore, you couldn't export else from a JS file, even if it's valid ReScript. So, the compiler prepends $$ to any identifier it thinks might be invalid. And via that, it also exports the identifier as $$<identifierName>. The alternative would be to not export those identifiers at all, which doesn't really make sense.
• However, default is a special case that might just be possible to support, because we're always adding a dedicated default export whenever we see default. So, even if $$default isn't exported anymore, we should be able to tweak the compiler to just refer to default where it now refers to $$default (done).