Add support for custom formulas.

This commit re-frames the formula concept used by the wave growth, swarm
amount, and boss health wave configuration properties. It fundamentally
changes how these values are calculated, from a static, compile-time set
of enum values and hardcoded expressions, to a powerful math expression
feature that supports constants, variables, operators, and functions.

In part to remain backwards compatible with existing MobArena setups,
and in part for a better user experience, the old enum-based expressions
are relocated into a new file, `formulas.yml`, as _macros_. The file is
written to the plugin folder if missing, and it contains a formula for
each of the legacy values for each of the enums. Additionally, it has a
global section with some predefined macros for inspiration's sake. The
goal of this file is to allow people to define new formulas and reuse
them in their wave configurations instead of having to duplicate the
same formulas again and again.

Parts of the system are extensible. It is possible for other plugins to
register additional constants, variables, operators, and functions.

Closes #460
Closes #461

changelog.md

@@ -11,6 +11,9 @@ These changes will (most likely) be included in the next version.
 
 
 ## [Unreleased]
+### Added
+- It is now possible to write custom formulas for wave growth in Default Wave, swarm amounts in Swarm Waves, and boss health in Boss Waves, allowing for much more control and fine-tuning. The formulas support various session-related variables as well as various mathematical operators and functions. Formulas can be predefined as macros in the new `formulas.yml` file. Check the wiki for details.
+
 ### Fixed
 - Arena signs in unloaded or missing worlds no longer break the startup procedure. Sign data is stored in a new format that MobArena will automatically migrate to on a per-world basis during startup.