fix: array methods + typed empty arrays

[IMisbahk]

What

• Added Array methods: .length() and .push()
• Allowed typed empty array assignment like var xs: [int] = []
• Added regression test src/tests/array_methods.rey

Why

Array methods should work uniformly across array types, and typed empty arrays should be usable without a placeholder initializer.

Notes

• Runtime implements Array.length()/Array.push(); typechecker enforces push element type for typed arrays.