feat(typings): use readonly arrays in parameters

[cherryblossom000]

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

Use readonly arrays in parameters. For example, these now work:

declare const guild: Guild
declare const positions: readonly ChannelPosition[]
guild.setChannelPositions(positions)

declare const member: GuildMember
declare const permissions: readonly PermissionString[]
member.permissions.has(permissions)

T[] is assignable to readonly T[], so the following will continue to work:

declare const positions2: ChannelPosition[]
guild.setChannelPositions(positions2)

declare const permissions2: PermissionString[]
member.permissions.has(permissions2)

Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.

[wasdennnoch]

This change breaks code where you're doing something like this:

const embed: MessageEmbedOptions = {
	fields: [{
		// fixed stuff
	}],
};
if (/* condition */) {
	embed.fields.push({
		// conditional stuff
	});
}

Because embed.fields is now marked as readonly you can't push into it anymore.

[cherryblossom000]

@wasdennnoch Hmm, I didn't think about that. I think this is a more common use case than using readonly arrays, so I've created #4794.