feat(ThreadChannel): add ThreadChannel#viewable

[suneettipirneni]

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

Closes #6969

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
• This PR changes the library's interface (methods or parameters added)

[Jiralite]

Whilst this pull request does fix the issue linked, it doesn't fix all instances of it.

Consider the scenario where a thread becomes locked or the client fetches a thread that happens to be locked without the manage threads permission. Then, consider the client fetching a message and running Message#editable. Through that, the result will be true but it should be false because the thread is locked and messages cannot be edited.

[suneettipirneni]

Whilst this pull request does fix the issue linked, it doesn't fix all instances of it.

Consider the scenario where a thread becomes locked or the client fetches a thread that happens to be locked without the manage threads permission. Then, consider the client fetching a message and running Message#editable. Through that, the result will be true but it should be false because the thread is locked and messages cannot be edited.

Good catch! I'm thinking even if it's locked and you have manage_threads perms editable should probably still be false, simply because the current state of the thread is still archived, and I believe editing a message doesn't implicitly unarchive it.

[Jiralite]

I'm thinking even if it's locked and you have manage_threads perms editable should probably still be false

Right.