User-facing `Term` and `Context` API

[multimeric]

I hope to craft my contexts programmatically, since I'm working with a format (RO-Crate) where the context must have a specific form. Therefore I'd like to propose these changes:

• Export Context and Term as public classes
• Convert Term to a dataclass or Enum class so we get typed fields
• Add documentation to the public methods of each of these classes
• Move Context._term_dict into Term.to_dict()
• Export the _ContextSourceType as ContextSourceType

Some initial work has already been done in #3169, but I'd like to check if this agrees with the vision for rdflib.