make exportmap procedures static instead of monkeypatching exportmap

[soryy708]

The following procedures:

ExportMap.get
ExportMap.for
ExportMap.parse

are conceptually static methods of ExportMap, but instead of using the static keyword they are monkey-patched onto the ExportMap class

ExportMap.get = function (...) {...}

which is surprising.

Lets make them static methods proper.

---

Tip for the reviewer/maintainer:

Use the "Hide whitespace" feature of GitHub's diff.

[ljharb]

thanks!

[ljharb]

you're right that this repo doesn't enforce that rule for some reason; i'll fix it shortly :-)