Add types for openseadragon (DefinitelyTyped#41279)

* feat: add `openseadragon` type definitions

* Augment test file, clean up tsconfig

* update collaborators & fix linting issue

* remove comment

* update events payload types

* add missing Viewport.imageToViewportRectangle method
update collaborators

* fix whitespace

* tweak: update default export

* chore: update required typescript version & remove node types ref

* tweak: update Options.tileSources types

* tweak: export as namespace &
update collaborators

* use declare namespace

Co-authored-by: Kristin Ruben <kristin.ruben@pathai.com>