function setAstral(on) {
    features.astral = on;
}
Enables or disables implicit astral mode opt-in. When enabled, flag A is automatically added to
all new regexes created by XRegExp. This causes an error to be thrown when creating regexes if
the Unicode Base addon is not available, since flag A is registered by that addon.

@private
@param {Boolean} on `true` to enable; `false` to disable.