feat(type): add string.base58 built-in validator

[WolfieLeader]

Summary

Closes #1520

Adds string.base58 as a built-in string validator following the existing hex pattern.

Base58 alphabet (per IETF draft and Bitcoin spec):
123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz
Excludes: 0 (zero), O (capital o), I (capital i), l (lowercase L)

• Regex: /^[1-9A-HJ-NP-Za-km-z]+$/ (uses + like hex, rejects empty strings)
• Registered via regexStringNode(), added to Scope.module and namespace $ type
• Tests individually verify each excluded character (0, O, I, l)

Note: the issue's suggested regex used \d which includes 0 — corrected to 1-9.

Test plan

• Valid Base58 strings (Bitcoin address format, simple alphanumeric) accepted
• Each excluded character rejected individually
• Empty string rejected
• pnpm prChecks passes (format, lint, typecheck, full test suite)

[pullfrog]

Clean, minimal addition. Regex correctly implements the Base58 alphabet (1-9A-HJ-NP-Za-km-z), properly excluding 0/O/I/l. Tests cover all excluded characters and the empty-string edge case. Follows the existing hex pattern exactly.

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[WolfieLeader]

Thanks pullfrog🐸