fix(GuildChannel): Fix manageable method for voice-channels

[DraftProducts]

Please describe the changes this PR makes and why it should be merged:

Resolves #6446

<GuildChannel>.manageable is not efficient with voice channels because <GuildChannel>.viewable is not checked on voice channels.

if (VoiceBasedChannelTypes.includes(this.type)) { 
   // connect permission check
 } else if (!this.viewable) { 
   return false;
 }

Status and versioning classification:

• I know how to update typings and have done so, or typings don't need updating

[ckohen]

This is incorrect, you can manage channels without permission to view them, the error in the original issue is caused because the bot did not have permission to view the channel, and thus could not change other roles permission on it.

[DraftProducts]

So you can manage voice-channel without view permission but for text channels, it is needed?

[ckohen]

Correct, I should have specified voice, but the permission bit to manage the channel is actually CONNECT on voice channels, regardless of the VIEW_CHANNEL bit. The error actually explains it better, you get Missing Access when you can't change overwrites on the channel, and Missing Permissions when you can't change one of the permissions you are trying to change

[kyranet]

Can't we just do this?

// Owner always can manage:
if (this.client.user.id === this.guild.ownerId) return true;

// Get the permissions, if we couldn't resolve to a member, return false:
const permissions = this.permissionsFor(this.client.user);
if (!permissions) return false;

// Get the bitfield and see if we fulfill it:
const bitfield = VoiceBasedChannelTypes.includes(this.type)
  ? Permissions.FLAGS.VIEW_CHANNEL | Permissions.FLAGS.MANAGE_CHANNELS | Permissions.FLAGS.CONNECT
  : Permissions.FLAGS.VIEW_CHANNEL | Permissions.FLAGS.MANAGE_CHANNELS;
return permissions.has(bitfield, false);

It's far more efficient, resolves MemberResolvable and creates a Permissions instance only once (as opposed to twice).

[iCrawl]

Are we moving forward with kyras suggestion here, or whats the plan @DraftProducts

[DraftProducts]

We can go with Kyras suggestion 👍