Lua: Allow defining and emitting new signal types from Lua scripts

[XHawk87]

Is your feature request related to a problem? Please describe.
It would be useful to be able to set up ruleset-specific events in Lua so that we can extend behaviour rather than modifying it directly.

Describe the solution you'd like

• Add a new signal.define(name) function that creates a new signal type that can be emitted from Lua.
• Add a new signal.emit(name, ...args) function that emits a signal in Lua for other functions to handle.

Additional context
Discussion in Discord
Discussion thread in Discord