Add Array#findIndex polyfill

[simonhaenisch]

Stencil version:

 @stencil/core@0.14.1

I'm submitting a:
[ ] bug report
[x] feature request
[ ] support request

Current behavior:

There is no polyfill for Array#findIndex.

Expected behavior:

There is a polyfill for Array#findIndex.

Other information:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex#Polyfill