fix: channelId on messageUpdate & honour partial objects

[austinhuang0131]

• https://github.com/revoltchat/revolt.js/commit/fe325e3628fb056591ad89fc9da4dbf08109a067 adds channel ID (which is supplied by the event) onto partial messages, for those who enabled partials in ClientOptions.
• https://github.com/revoltchat/revolt.js/commit/3ffa0e79e7f0fed6caeb4711c0542be9cdd7d9a8 makes partial properties actually persist in storage (otherwise isPartial is useless).
• https://github.com/revoltchat/revolt.js/commit/16b1e60cb2a402d6c9d015fdffdf796b36d6d6be allows partial objects to be converted to full objects whenever the respective fetch methods are called.

Please make sure to check the following tasks before opening and submitting a PR

• [x] I understand and have followed the contribution guide
• [x] I have tested my changes locally and they are working as intended