Function isMinimalWritableStore

isMinimalWritableStore(store): boolean
Provides a basic test for a given variable to test if it has the shape of a minimal writable store by having a subscribe and set functions.

Note: functions are also objects, so test that the variable might be a function w/ subscribe & set functions.
Parameters
- store: any
  variable to test that might be a store.
Returns boolean
Whether the variable tested has the shape of a MinimalWritable store.