Create a new top-level operator AlwaysTrueMatchExpression that will always match. For AlwaysFalseMatchExpression, we will re-purpose the existing FalseMatchExpression and change the way it serializes.

Proposed Syntax

Both operators are top-level operators that take no path, and whose only argument is 1:

{ $alwaysTrue: 1 }
{ $alwaysFalse: 1 }