Added missing SPACE char before keyword extends

[kozi]

in function writeInterface()

#88

[bvallee-thefork]

We're affected by this too (and https://github.com/fossamagna/google-script-dts-generator/pull/194 seems to be affected too).

@RyanCavanaugh Can this be merged?

This bug results in interfaces with extends being generated like this since 3.7.0: export interface MyInteraceextends MyOtherInterface{

[AmitFeldman]

Also affecting our project, why not merge this fix? @RyanCavanaugh