Enable branching type inference in union type param

[kwonoj]

This PR adds small amend for discussions enabling type inference for union type branches.

Few notes:

why not global-augument?

: since lodash.isFunction is untyped module, so no augumentation is possible.

name typedIsFunction

: I just picked random name for this, can apply any way you'd prefer. To preserve name, you could think to use require like below instead of import syntax.

const isFunction: (val: any) => val is Function = require('lodash.isFunction')....

[anaisbetts]

We should probably export all of the isXYZ methods from utils, I'll have a look. Thanks for the val is BLAH tip!

[kwonoj]

@paulcbetts yup, feel freely close PR. this is demonstration purpose and code might need to be updated as needed.