Switch to symbols to namespace props

[SimonMeskens]

I'm waiting for 2.9 to go stable to switch props from unique strings to symbols. From cursory testing, this works out of the box, but it probably needs the new keyof mechanics in 2.9 to work properly.

[SimonMeskens]

The documentation for interface TypeProps should mention that symbols are preferred

[KiaraGrouwstra]

Word of warning: many type level operations like Omit currently cannot deal with symbols as they have no equivalent to keyof.

[SimonMeskens]

@tycho01 2.9 keyof supports symbols and numbers directly, from cursory testing, it handles TypeProps just fine.