Introduce internal namespace and remove api namespace. (#8443)

Introduce internal namespace and remove api namespace. (#8443)

The public cvc5 API now lives in the cvc5 namespace. All internal parts were moved into the (new) internal namespace.
The final hierarchy will be as follows:

cvc5
  ~~ public API
  ::context
  ::internal
  ::parser
  ::main

After this PR it will be:

cvc5
  ~~ public API
  ::internal
      ::context
      ::main
  ::parser