Check for invalid literals as ouput of parsers

[pchampin]

The current Turtle file is valid according to the grammar:

[] <tag:> "foo"@abcde .

However, abcde is not a valid BCP47 language tag, so the produced graph is not valid according to the RDF abstract syntax.

Currently, Sophia blindly trusts the output of the parsers, creating the corresponding language rag with LanguageTag::new_unchecked, which results in a value violating the contract of its type.

The output of parsers should therefore be checked, at least for the language tags.

[pchampin]

A similar issue, which I raised here, occurs when a literal is explicitly typed as rdf:langString but has no language tag. This also should be checked -- unless Turtle (and other concrete syntaxes) are changed to reject it.

[pchampin]

False alarm... abcde is in fact a valid BCP47 tag, my mistake.
On the contrary, abcdefghi is an invalid tag, and is correctly rejected by all Sophia parsers.

"foo"^^rdf:langString may still be an issue, though, pending the decisions of the RDF 1.2 WG.