refactor: make public builder props getters

[suneettipirneni]

This is a replacement for the previous pr since gh is having trouble loading the diffs.

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

Stores raw api data in a data object and allows access to builder properties via getters.

Rationale

There's quite a few benefits in using getters for builders:

• Currently builders relies on Reflect#set which is much slower than direct prop assignment, in one benchmark I did it was waaay slower than direct assignment. Beyond performance Reflect#set is less typesafe since property keys are strings and it allows values to be set as any. This PR removes the usage of these methods, and preserves the readonly nature of the fields.

• The classes are now more flexible. For example, right now if we decided to add a field to any builder class, it would be outputted in toJSON simply because toJSON just spreads this: { ...this }. In addition, since we now have getters we can do any computation before a value is returned unlike a regular field.

• Constructor assignment is easy, in some cases it's just assigning this.data = data. toJSON is straightforward as well, as it's essentially just returning a copy of this.data.

• Allows for idiomatic js/ts class field naming. The props are simply retrieved from this.data so they're not constrained to any naming conventions used by the API.

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)

[kyranet]

Just one last thing, then LGTM.

[vladfrangu]

This needs a rebase

Stale