fix($parse): mark constant unary minus expressions as constant

Request Type: bug

How to reproduce: Parse an expression like `-42`. Check its `constant` flag. Expected: `true`. Actual: `false`.

Component(s): $parse

Impact: small

Complexity: small

This issue is related to: 

**Detailed Description:**

Expressions that use the unary `-` operator are currently never marked as constants.

Make `Parser.ZERO` a constant expression function, so that unary minus
expressions that use it may be marked constant.

**Other Comments:**