Add callback-based Array prototype methods

[memburg]

What changed

• Added Array.prototype.forEach, map, filter, reduce, some, every, find, and findIndex to runtime type methods
• Added a callback invocation bridge so type-level builtins can execute user and builtin callbacks
• Added specs for happy paths, edge cases, and argument validation for the new methods
• Updated reference docs to mark these Array methods as available

Validation

• Ran crystal spec

Closes #2