feat: add fromScalar() for creating 0-dimensional scalar arrays - #28

This PR adds NDArray::fromScalar() to create 0-dimensional arrays from a single PHP value.

Motivation and Context

The codebase already supports 0-dimensional arrays internally (e.g. from reductions on scalars), and has toScalar() to extract the value back, but there was no symmetric factory method to create a 0-d array from a PHP value. Users had to resort to NDArray::full(42, []) which is unintuitive.

What's Changed

• Added NDArray::fromScalar(bool|Complex|float|int $value, ?DType $dtype = null): self — delegates to full() with shape []
• Added from_scalar() global function alias in Functions.php
• Added 7 test cases covering int, float, bool, Complex, explicit dtype, broadcasting, and the global function
• Added documentation to the quick-start guide

Breaking Changes

None.