Fix zen-observable import

[MatthiasKunnen]

The old import causes the following error:

node_modules/@apollo/client/utilities/observables/Observable.d.ts:1:8 - error TS1259: Module '".../node_modules/@types/zen-observable/index"' can only be default-imported using the 'allowSyntheticDefaultImports' flag

1 import Observable from 'zen-observable';
         ~~~~~~~~~~

  node_modules/@types/zen-observable/index.d.ts:69:1
    69 export = Observable;
       ~~~~~~~~~~~~~~~~~~~~
    This module is declared with using 'export =', and can only be used with a default import when using the 'allowSyntheticDefaultImports' flag.

[apollo-cla]

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[MatthiasKunnen]

Hmmm, the correct import style does not seem to be compatible with how apollo-client is using zen-observable. It all originates from the usage of esModuleInterop. I reckon I'd have to until rxjs replaces zen-observable in this repository (#5749)?

[benjamn]

Observable is supposed to be a constructor function, but import * as anything from "wherever" will always give you a (module namespace) object for anything, so I don't think this change is quite correct/enough by itself.

[MatthiasKunnen]

@benjamn, you're right. I'll enable allowSyntheticDefaultImports to fix this problem for now. That being said, it'd be great if installing apollo-client would not require me to change my tsconfig.