[Enhancement] Support a "label" command

[Deokishisu]

One thing that has come up a handful of times that I would like to see from poryscript is the ability to put labels anywhere without defining a new script. While scripting there are a few very infrequent circumstances where being able to jump to an arbitrary point in a script without using one of the loops and without duplicating script segments is helpful.

• Me, from the Discord.

[huderlem]

This is now in master. Before closing the issue, I'm waiting for confirmation that it works as expected and is actually useful.