Feature/mod framework

[flend]

• Added Rapid Brogue v1.5 variant and ability to switch to variant at run-time (when game not in progress). Switching variants currently implemented on the command line at game startup only.
• Adds src/variants. Each variant defines a struct of game constants, its own table variables, and a function to switch the global variables for constants and tables pointers to the variant's own.
• Many #defines replaced with variant-specific game constants.
• Globals files tidied up and globals made const where possible. Many color functions now accept const colors.
• Implemented hard by-level item gen and charm minimum recharge duration, used in Rapid Brogue variant.
• Game variants have their own high scores file.
• Variant versions are saved in the recording header, core game version is not saved (since variant version determines recording compatibility).

[tmewett]

Ok, this is too big to stay unmerged for any longer, and it's in a good shape.

Big props for tackling this, we've theorycrafted about it for a while but no one got it across the line until now 🚀