fix(User): fix bot and system properties being incorrect in some cases

[ImRodry]

Please describe the changes this PR makes and why it should be merged:
Due to the system property only being present in user objects when the user is a system user, it would end up always being null instead of false like it should be. This PR fixes that so that it's true for system users, false for non-system users and null for partial users (regardless of whether they're system or not, since we don't know that)
Another issue in the User class was that the bot property would always be false for partial users due to the check typeof this.bot !== 'boolean' which would return true since this.bot was set to null previously. This fixes that so that it's null for partial users, true for bots and false for regular users.

⚠️ I did test these changes but only by creating partial objects myself since I couldn't find a way to easily obtain an actual partial user object. Please let me know if there's something that could go wrong that I didn't pick up in my tests.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

[SpaceEEC]

When patching an already cached user with partial data (happens in audit logs and wherever we'd get a partial user in the future), the bot and system property would be assigned false.

Can be reproduced by the following steps:

• Enable the USER partial
• Add/remove a role from/to a bot
• Fetch audit log
• Inspect User#bot of that bot and find false

Same thing should apply to system, but I don't have a system user to assign a role to at hand 😄

The ideal behavior of structure._patch({}) for potentially partial structures would be no changes. (Although this is not yet the case for all instances, but we are slowly getting there.)

[ImRodry]

When patching an already cached user with partial data (happens in audit logs and wherever we'd get a partial user in the future), the bot and system property would be assigned false.

Can be reproduced by the following steps:

• Enable the USER partial
• Add/remove a role from/to a bot
• Fetch audit log
• Inspect User#bot of that bot and find false

Same thing should apply to system, but I don't have a system user to assign a role to at hand 😄

The ideal behavior of structure._patch({}) for potentially partial structures would be no changes. (Although this is not yet the case for all instances, but we are slowly getting there.)

I tried to reproduce this and got the same behavior but that's because the user is not classified as a partial since it's username is a string (at least it was in my case).
I believe this happened because bot was not present in the data used to create the user but the partial property returned false so it's technically the expected behavior. I believe partial should have more checks than just the type of the user's username to prevent situations like this.

[ImRodry]

The previously mentioned issue should be fixed as I made it so that it takes both the received data and the existing data for the user into account. So if one isn't present, the other one will be kept instead of being overwritten with false for non-partial users. Please let me know if there are any other issues with this approach

Stale