feat(Guild): add systemChannelFlags (#3559)

* Add systemChannelFlags bitfield to Guild

* Implement @vladfrangu's suggestions

* fix: apply suggestions, reverse order of flags, reword docs

* docs: add SystemCHannelFlagsResolvable typedef

Co-authored-by: SpaceEEC <spaceeec@yahoo.com>

src/util/SystemChannelFlags.js

@@ -0,0 +1,33 @@
+'use strict';
+
+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;