Return Diagnostic#rule as Symbol

[st0012]

Problems

• Rubydex::Diagnostic#rule is typed as a Symbol, but diagnostics were built with string rule values.
• That mismatch makes runtime behavior disagree with the Ruby-facing API contract.

Solutions

• Intern native diagnostic rule strings before constructing Rubydex::Diagnostic objects.
• Update the parse-error diagnostic test to assert symbol rule values.

[vinistock]

What do you think of changing the code to return a symbol instead? It seems nicer for these unique labels