FlatExpression: add SE-mirroring type-inferring factory overloads

[Copilot]

ExpressionTree factory methods required an explicit Type argument everywhere, making it impossible to swap Expression.Add(a, b) for tree.Add(a, b) without callers threading types manually.

Changes

• Type-inferring arithmetic overloads — Add, Subtract, Multiply, Divide, Modulo infer result type from the left operand (NodeAt(left).Type); explicit-type overloads preserved
• Type-inferring unary/assign — Negate(operand) infers from operand; Assign(target, value) infers from target
• Generic Binary without explicit type — Binary(nodeType, left, right) infers from left; explicit-type variant unchanged
• Missing comparison helpers — NotEqual, LessThan, LessThanOrEqual, GreaterThan, GreaterThanOrEqual → typeof(bool); AndAlso, OrElse → infer from left (matches SE behaviour for user-defined operators; for bool operands the inferred type is already bool)

var tree = default(ExpressionTree);
var px = tree.Parameter(typeof(int), "x");
var py = tree.Parameter(typeof(int), "y");

// Before: explicit Type required everywhere
var add = tree.Add(px, py, typeof(int));

// After: mirrors Expression.Add(px, py) — type inferred from left operand
var add = tree.Add(px, py);
var lt  = tree.LessThan(px, py);   // returns bool, no type arg needed