Implement narrowType for ExpressionCompare with ExpressionCompare::EQ with two operands.

$eq: ["$a", "string"] -> makes it string(S)
$eq: ["$a", 1] -> makes it number(S)
$eq: ["$a", null] -> makes it null|missing.

Whether the equality holds is given by assumeTrue, e.g.
bool equalityHolds = (expr->getOp() == ExpressionCompare::EQ) == assumeTrue;

Note:

• BSON comparison applies (equality to double(1) will match int(1), so that should just become the number(S) type)