dnsimple / dns_erlang / 20237295847

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Fix Base64 lexing of zone files (and add support for ZONEMD records) (#94)

This PR introduces support for the `ZONEMD` resource record ([RFC8976](https://datatracker.ietf.org/doc/html/rfc8976)) and fixes a lexing issue where Base64-encoded strings containing + or / were failing to parse correctly.

1. New Record Support: ZONEMD
    - Added ZONEMD to the supported resource record types: this allows the parser to handle Message Digests for DNS Zones ([RFC8976](https://datatracker.ietf.org/doc/html/rfc8976)), enabling cryptographic verification of zone data integrity.

2. Lexer Fix: Base64 Support
    - Updated `LABEL_CHAR` and `LABEL` regex definitions in `src/dns_zone_lexer.xrl` to include `+` and `/`: previously, the lexer was restricted to alphanumeric characters (plus -_*:) for unquoted strings. This caused failures when parsing Base64-encoded fields—common in `DNSKEY`, `RRSIG`, `DS`, and now `ZONEMD` records—if they contained standard Base64 characters `+` or `/`.

Context & References
- [RFC8976](https://datatracker.ietf.org/doc/html/rfc8976) (Message Digests for DNS Zones): Defines the ZONEMD record to provide a cryptographic digest of the zone's content.
- [RFC4648](https://datatracker.ietf.org/doc/html/rfc4648) (Base64 Encoding): Defines the Base64 alphabet as A-Z, a-z, 0-9, +, and /.
- [RFC1035](https://datatracker.ietf.org/doc/html/rfc1035) §5.1: Defines Master File RDATA as "character-strings", which should allow contiguous sequences including + and /.

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