Add new permissions

[Earlopain]

First commit makes the numbers bitflags which I find easier to parse and follows all the other flags in the repo. I also added docs for them.

Second commit adds missing flags currently documented at https://discord.com/developers/docs/topics/permissions#permissions-bitwise-permission-flags

Third commit renames some fields to what they are now called on discords docs page.

Fourth commit adds currently undocumented flags, but there is a pending pr for those at https://github.com/discord/discord-api-docs/pull/6305

[Earlopain]

The 2 permissions I added in the last commit are no longer undocumented, the PR for that has been merged.

[mrbentarikau]

just a thought, maybe moving all these permissions from structs.go to a new file, like permissions.go?