Unary minus has precedence over exponentiation (ie. incorrect parsing of −x²)

In filtrex v2, the syntax `-x^2` is interpreted as `(-x)^2`, not as `-(x^2)`. This is contrary to the common order of operation and will be fixed in version 3. Thanks to @​rici for [pointing this out](https://stackoverflow.com/a/68061276/1137334).