rework EvalString function, to return Go-Compatible types

[nitram509]

Motivation

currently e.g. feel.Number is returned, when evaluating "4 + 5".
such evaluation should return a Go type e.g. int to ease usage.

Hint: some tests are in place and skipped - search for t.Skip("method implemented, but return type should be native Go type")
to get some examples.

[trojanc]

How should this conversion happen... An easy assumption might just be to return all numbers as float64 which should suite all numbers...

When a Number is created from a Scope where the underlying type is still available we could keep track of it in the Number struct

But as soon as math has been done or the number is parsed from text, the type becomes unclear...

This is a very primitive solution that at deals with number and null

func EvalStringWithScope(input string, scope Scope) (any, error) {
	ast, err := ParseString(input)
	if err != nil {
		return nil, err
	}
	intp := NewIntepreter()
	if scope != nil {
		intp.Push(scope)
	}
	r, err := ast.Eval(intp)
	if err == nil && r != nil {
		switch r.(type) {
		case *Number:
			n := r.(*Number)
			if i64, acc := n.v.Int64(); acc == big.Exact {
				return i64, nil
			}
			return r.(*Number).Float64(), nil
		case *NullValue:
			return nil, nil
		}
	}
	return r, err
}

Are there any specification rules that say in what data type the number must be returned?

[trojanc]

I made #26 let me know if that's heading in the right direction...