create-react-app doesn't support mjs

[Kamahl19]

Updating to 1.1.0.beta-8 doesn't work on CRA based apps because of Can't import the named export 'createContext' from non EcmaScript module (only default export is available)

This is not really a problem of SWR but if we want to use this package on CRA based apps, the mjs has to be reverted.

Additional context facebook/create-react-app#10356

Other libs have the same issue formatjs/formatjs#1395