chore: enable TS exactOptionalPropertyTypes

[pskelin]

TypeScript description of the flag: https://www.typescriptlang.org/tsconfig#exactOptionalPropertyTypes

This change is a preparation for introduction of property initializers. The goal is when a component property is defined as optional with a default value prop?: string = "default value"

This will be interpreted such that the value can be left out when using it from JSX (and it will get the default), but assigning undefined is forbidden and not handled by the component.

Enabling exactOptionalPropertyTypes is a preparation inside the project for this usage.