src: support new message fields (#3388)

* src: Update channel pattern

* src: Remove useless non-capture group

* src: it's as though we're starting fresh

* src: Bring this up to date for reals now

* src: typings and a bug fix

* src: Add crossposted channels to message mentions

* src: Requested changes and add typings

* src: Move Object.keys outside loop

* typings: Fix enum being exported when it shouldn't

* src: Consistency with roles and users

* docs: Correct docstring for MessageFlags#flags

* docs: Correct docstring for MessageMentions#crosspostedChannels

* docs: Suggestions
Co-authored-by: SpaceEEC

* src: Reset flags to 0 if no flags are received on MESSAGE_UPDATE

src/util/MessageFlags.js

@@ -0,0 +1,25 @@
+'use strict';
+
+const BitField = require('./BitField');
+
+/**
+ * Data structure that makes it easy to interact with an {@link Message#flags} bitfield.
+ * @extends {BitField}
+ */
+class MessageFlags extends BitField {}
+
+/**
+ * Numeric message flags. All available properties:
+ * * `CROSSPOSTED`
+ * * `IS_CROSSPOST`
+ * * `SUPPRESS_EMBEDS`
+ * @type {Object}
+ * @see {@link https://discordapp.com/developers/docs/resources/channel#message-object-message-flags}
+ */
+MessageFlags.FLAGS = {
+  CROSSPOSTED: 1 << 0,
+  IS_CROSSPOST: 1 << 1,
+  SUPPRESS_EMBEDS: 1 << 2,
+};
+
+module.exports = MessageFlags;