Convert type assertion expressions to NSExpression

[1ec5]

The NSExpression-based property value implementation in #10726 should be able to express the type assertion operators defined in the style specification:

• array
• boolean
• number
• string

The coercion operators corresponding to these operators (literal, to-boolean, to-number, and to-string) have already been implemented.

Additionally, typeof needs to be implemented.

[1ec5]

All these operators can now be expressed by the last-ditch MGL_FUNCTION function. In practice, there should be little need for iOS and macOS developers to explicitly use these operators, compared to more ergonomic, built-in CAST operator.