Remove window declarations from pure types

[fruchtose-stripe]

Summary & motivation

This PR fixes an issue from this comment: #543 (comment)

Both pure/index.d.ts and dist/index.d.ts attempt to modify the Window class. The types/pure.d.ts imports loadStripe from types/index.d.ts, and the latter module monkeypatches Window. The pure module then receives both the function type it wants, as well as the monkeypatched Window class.

I have fixed the issue by moving the loadStripe declaration into a new shared.d.ts module.

Testing & documentation

I built the types and verified that Window is no longer monkeypatched in the pure output types.