feat(Channel): add isText() type guard

[vaporoxx]

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

Currently, there is no easy way in TypeScript to find out if a channel is text-based or not.

['text', 'dm', 'news'].includes(channel.type)
channel.type === 'text' || channel.type === 'dm' || channel.type === 'news'

Both of these examples work fine during runtime, but don't act as a type guard; If channel had the type Channel before the check, it will also keep it afterwards.

channel instanceof TextChannel || channel instanceof DMChannel || channel instanceof NewsChannel
channel as TextChannel

These two would actually narrow the type down to only text-based channels, but both aren't really optimal. The former option is too long and the latter doesn't actually check the type, it just tells the compiler that it will be a TextChannel even if it may not be one.

I generally think that checks like this should be able without having to either import the classes and doing multiple instanceof checks or casting its type which may lead to side effects.

Status

• Code changes have been tested against the Discord API
• I know how to update typings and have done so

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.

[kyranet]

Just an alternative that also works:

[advaith1]

I feel like naming this isText is misleading, maybe isTextBased or similar?

[iCrawl]

How so? Channel#isText can mislead to what assumption?

[advaith1]

it sounds like it only checks if it is a text channel, and that it should be false for news channels

[HarmoGlace]

For "normal" people a news channel is still a text channel. But yes, it is exact to say it is a text-based channel

[advaith1]

In the UI it is treated as a seperate channel type, as it is

[iCrawl]

Sure, but I don't see how isTextBased is any better than isText.

It's both not explaining that a news channel is included, especially not in/with that UI argument.

[HarmoGlace]

Also, why is it a method named isText instead of a getter named text ? Because all bolean getters in the library are not named by starting with is

[vaporoxx]

Getters count as properties and X is Y is only valid for function return types.

[HarmoGlace]

I know it is counted as a property but I don't see what you mean, e.g. TextChannel#viewable returns a boolean and it is not named isViewable https://discord.js.org/#/docs/main/stable/class/TextChannel?scrollTo=viewable so I don't see why it would not be named text instead of isText

[vaporoxx]

If you have a look at the changes for index.d.ts, you'll see that it's not a regular boolean but instead a type guard so that you can use TextBasedChannel methods on it. That type is only available as function return types so it can't be added as a getter.