Create redirect(s) for old links

[ExE-Boss]

@bterlson In order for old links to https://bterlson.github.io/ecmarkup/ to work, it’s necessary to create https://github.com/bterlson/bterlson.github.io as per https://gist.github.com/domenic/1f286d415559b56d725bee51a62c24a7.

See also:

• https://github.com/tc39/proposal-export-default-from/pull/9