feat(Guild): add support for system channel flags (#3793)

src/util/SystemChannelFlags.js

@@ -0,0 +1,31 @@
+const BitField = require('./BitField');
+
+/**
+ * Data structure that makes it easy to interact with a {@link Guild#systemChannelFlags} bitfield.
+ * <info>Note that all event message types are enabled by default,
+ * and by setting their corresponding flags you are disabling them</info>
+ * @extends {BitField}
+ */
+class SystemChannelFlags extends BitField {}
+
+/**
+ * Data that can be resolved to give a sytem channel flag bitfield. This can be:
+ * * A string (see {@link SystemChannelFlags.FLAGS})
+ * * A sytem channel flag
+ * * An instance of SystemChannelFlags
+ * * An Array of SystemChannelFlagsResolvable
+ * @typedef {string|number|SystemChannelFlags|SystemChannelFlagsResolvable[]} SystemChannelFlagsResolvable
+ */
+
+/**
+ * Numeric system channel flags. All available properties:
+ * * `WELCOME_MESSAGE_DISABLED`
+ * * `BOOST_MESSAGE_DISABLED`
+ * @type {Object}
+ */
+SystemChannelFlags.FLAGS = {
+  WELCOME_MESSAGE_DISABLED: 1 << 0,
+  BOOST_MESSAGE_DISABLED: 1 << 1,
+};
+
+module.exports = SystemChannelFlags;