fix(BitField): ensure missing returns an array of strings

[SpaceEEC]

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

Currently BitField#missing assumes the passed bits are, if an array, an array of strings, this is not necessarily correct.
For example

const missing = new Permissions([Permissions.FLAGS.SEND_MESSAGES, Permissions.FLAGS.VIEW_CHANNEL])
    .missing([Permissions.FLAGS.VIEW_CHANNEL, Permissions.FLAGS.BAN_MEMBERS]);
console.log(missing); // [ 4n ] instead of [ "BAN_MEMBERS" ]

This PR ensures that BitField#missing always returns an array of strings.

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

[kyranet]

This code just looks so much more efficient, I like it.