Fix Util#splitMessage when destructured

[BannerBomb]

Please describe the changes this PR makes and why it should be merged:
this PR fixes the Utils#splitMessage method because resolveString should be called as Util.resolveString since it's a static method.

---

Update
kyranet posted a better reason why this PR should be merged here.

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.

[monbrey]

What's the point of this exactly? The method wasn't broken, and this refers to Util already, as per MDN

this would be undefined if the method is deconstructed from the class, so I now see the use case for this

[kyranet]

I have to note, though, your reason for the fix is incorrect. The method works fine with this, it refers to the class itself (aka the Util class) as it's in a static method, as well as if it was an instance method, it would refer to the instance itself.

What breaks this is when you destructure the method, as in:

const { util: { splitMessage } } = require('discord.js');
splitMessage('');
// TypeError: Cannot read property 'resolveString' of undefined

But if you do not destructure the method itself, it will work just fine:

const { util } = require('discord.js');
util.splitMessage('');
// [ '' ]

That's because when destructuring the method, you're removing it's context, used for the this value, hence the error. I believe this PR should be merged as this problem makes the method inconsistent with all the others, which don't error when destructured.