Preparations for eliminating arithmetic subtyping (#7637)

Preparations for eliminating arithmetic subtyping (#7637)

Adds TypeNode::isArithmetic, NodeManager::mkConstReal and NodeManager::mkConstInt.

The first method (for now) is equivalent to TypeNode::isReal, and the latter 2 are equivalent to NodeManager::mkConst(CONST_RATIONAL, ...).

This furthermore distinguishes all uses of isReal: all that intend to be isArithmetic are changed in this PR. Redundant uses of isReal() || isInteger() are merged to isArithmetic().

Due to the above, there are no behavior changes in this PR.