Added type definitions for @scalableminds/prop-types

[hotzenklotz]

I added the Typescript types definitions for the @scalableminds/prop-types package. Since we forked the package, we can not just install @types/prop-types. Therefore, I copied the type definition (MIT license) and adjusted them to fit out package structure.

Steps to test:

• Run typechecker

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(Please delete unneeded items, merge only when none are left open)

• Updated changelog
• Updated migration guide if applicable
• Updated documentation if applicable
• Adapted wk-libs python client if relevant API parts change
• Removed dev-only changes like prints and application.conf edits
• Considered common edge cases
• Needs datastore update after deployment

[philippotto]

awesome 👍