(dns) Highlight CAA property tags (issue, issuewild, iodef, …) #4475

Context

Follow-up from #3768 (DNS zone grammar improvements: strings, classes, TXT handling).

CAA records look like:

@  IN  CAA  0  issue  "letsencrypt.org"
@  IN  CAA  0  issuewild  ";"
@  IN  CAA  0  iodef  "mailto:security@example.com"

The property tags (issue, issuewild, iodef, and any other standard CAA tags worth listing) are currently plain text. It would be nice to scope them as something distinct (e.g. attr or similar) so they stand out from the flag and the value string.

Suggested approach

• Prefer matching only in a CAA context (after the CAA keyword / on CAA RDATA), not bare issue anywhere in the file.
• Keep the set small and explicit (known tags from the CAA RFCs / IANA registry as needed).
• Add a small markup fixture under test/markup/dns/.

References

• https://datatracker.ietf.org/doc/html/rfc8659
• PR: edit DNS grammar #3768

Good first language-grammar task if you want to touch src/languages/dns.js.