feat: add string.isbn keyword for ISBN validation

[Ojoxux]

Summary

Following the precedent set by string.creditCard, this PR introduces string.isbn as a built-in validator.

I've encountered the need for this in my own projects before, where I ended up relying on custom validation.

Validating ISBNs requires more than a simple format check, since ISBN-10 and ISBN-13 use distinct checksum algorithms:

• ISBN-10 uses a modulo-11 check and allows X as a valid check digit.
• ISBN-13 uses a modulo-10 checksum with alternating 1/3 weights.

As a result, rolling a custom implementation with narrow can be easy to get subtly wrong.

Behavior:

• Accepts ISBN-10 or ISBN-13
• Allows hyphen/space separators, which are stripped before validation
• Validates the appropriate checksum
• Rejects lowercase x as a check digit

Examples:

Input	Result
"978-4-87311-873-4"	✅ valid
"4-87311-873-5"	✅ valid
"020530902X"	✅ valid
"9784873118735"	❌ invalid checksum
"020530902x"	❌ invalid

Implementation notes:

• Adds the validator in ark/type/keywords/string.ts
• Exposes it via rootSchema({ domain: "string", predicate: ... })
• Registers it in Scope.module
• Adds the corresponding namespace $ type

Test plan

• Valid ISBN-10 and ISBN-13 inputs are accepted, with and without separators
• Invalid checksum, lowercase x, wrong length, and non-digit input are rejected
• pnpm prChecks passes