This commit adds support for the IRCv3.2 MONITOR command [1]. This
includes a new function for sending MONITOR commands to the IRC server
and constants for creating handlers for the server responses.
Discussion: Currently no fancy abstraction is offered for the MONITOR
command. Meaning the caller can:
1. Send MONITOR commands to the server with invalid modifier, where
`modifier ∉ {+, -, C, L, S}`.
2. Add a target to the list of targets being monitored which is already
in that list. Which the client MUST NOT do according to the
specification.
Both 1. and 2. could be addressed by implementing separate functions for
the different MONITOR command modifiers (e.g. `MonitorAdd()`,
`MonitorDel()`, …). If 2. is not considered an issue and should be
resolved by the caller 1. could alternatively by defining a new type for
the modifier.
An entirely different solution would be adding methods for monitoring to
the `User struct`. For example: `User.Monitor(), User.Unmonitor(), …`.
[1]: https://ircv3.net/specs/core/monitor-3.2.html
