Add toJSON() to Filter base class, implement or throw within FunctionFilter

[amcclain]

We can't actually serialize a function filter, but having toJSON() on the base class would be helpful and avoid cheats like:

hoist-react/data/filter/CompoundFilter.ts

Line 51 in c202046

filters: this.filters.map((f: any) => f.toJSON()),

which could then throw in an unhandled way.

Should we implement something human-readable, even if it could not be parsed back?