isupport.PREFIX is not ordered

[half-duplex]

We don't currently use isupport.PREFIX for anything interesting, but the IRC spec says it's in a specific order, #2197 will need it to preserve the order, and we currently shove it into a normal dict. See https://modern.ircdocs.horse/#prefix-parameter

[half-duplex]

In python3.7 normal dicts seem to require preserving insertion order, but oops, even in 8.0 we only require 3.6+, where preservation is only an implementation detail of cpython.
Either way, it needs to be tested and documented.