test for function with `typeof` not `instanceof`

[RamIdeas]

Allows environments (e.g. jest) to override the global Function constructor

Checking via instanceof resolved to false, which caused the Path#source property to be set incorrectly where partialTest would return {} rather than null when it wasn't meant to match

EDIT: actually, not sure if it is jest which is wrapping Function or the environment I am running my code in (Cloudflare Workers) which doesn't allow eval/new Function

[kcirtaptrick]

This change would be great, currently preventing this package from use in next.js middleware since it disallows references to Function. This is probably also true for all edge functions