This is a simple extension of GenericLexer, and is used in more than
just LibXML, so let's move it into AK.
The move also resolves a FIXME, which is removed in this commit.
I got fed up with looking at error messages that tell me "VERIFICATION
FAILED: !is_error()". So this commit introduces DiagnosticEngine class
whose purpose is to accumulate and print more user-friendly errors.
Currently, they are not extremely useful, but the plan is to store
all function-local state in JSSpecCompiler::Function and all
"translation unit" state in ExecutionContext.