feat(SlashCommands): add number option type

Please describe the changes this PR makes and why it should be merged: This PR adds support for the number option type as is specified here.

close #22

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)

discordjs / builders