docs: correctly type getters

[IRONM00N]

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

• made all getters be typed correctly as getters - this improves typescript error messages and makes it clearer to the end-user that the value is calculated
• remove GuildChannel#calculatedPosition from the typings since it doesn't exist in the source code
• corrected a few properties that should(n't) have been marked as readonly

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 only includes non-code changes, like changes to documentation, README, etc.

[kyranet]

This was a task that I wanted to get done for a while, thanks! 🙏🏼