GetDirective gets a directive by name.

GetDirectiveArg returns nil if the argument was not found.

GetDirectiveArgBool returns `(false, false, nil)` if directive or argument was not found, or returns an `error` if it was found but it was not a boolean.

GetDirectiveArgEnum returns `false` if directive or argument was not found, or returns an `error` if it was found but it was not a string.

GetDirectiveArgStr returns `false` if directive or argument was not found, or returns an `error` if it was found but it was not a string.

GetDirectives returns nil if the node does not have directives.

SetDirectives returns false if the node does not accept directives.