Improve AgnosticBaseRouteObject type for projects using exactOptionalPropertyTypes

[AsamK]

Projects that use typescript with the exactOptionalPropertyTypes=true setting see a type error when using react-router.
The workaround of using skipLibCheck=false is not desirable as it weakens type-system accuracy for the whole project. https://www.typescriptlang.org/tsconfig/#skipLibCheck

This PR makes the minimal changes to work for projects using exactOptionalPropertyTypes=true and would be immensely helpful

Fixes #10392
Fixes #11991
Fixes #14504

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