fix(Typings): Fix BitField toJSON/valueOf return types

[wasdennnoch]

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

In #4879 BitFields were updated to support other value types such as BigInts, however some of the typings weren't updated accordingly.
Reference:

discord.js/src/util/BitField.js

Lines 117 to 123 in f5f3f77

	toJSON() {
	return typeof this.bitfield === 'number' ? this.bitfield : this.bitfield.toString();
	}
	valueOf() {
	return this.bitfield;
	}

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