[mysql] Add support for the symbolic unary 'not' operator

Addressing https://github.com/FoundationDB/sql-parser/issues/105, this pull request adds support for the symbolic unary 'not' operator that is unique to the [MySQL expression grammar](https://dev.mysql.com/doc/refman/5.0/en/expressions.html). It follows the pattern set by the bitwise and arithmetical operators, promoting `UnaryLogicalOperatorNode` to a concrete class and extending it to permit both syntaxes. This factoring preserves the existing semantics of the `NotNode` subclass (used exclusively for the `NOT` keyword). Visitors that traverse expressions were extended to support the new operator (`BooleanNormalizer` and `TypeComputer`).

The feature flag `MYSQL_SYMBOLIC_NOT_OPERATOR` is enabled by default, but not strictly necessary.

/cc @pcarrier