Enable on-demand heal and mods

[jwalk-511]

Allow players to add any mod type or trigger a heal on demand.

This enables them to replicate abyss cards, the effects of eating food, or other event buffs.

Note: if both this and Expanded Booking are approved, the refactor of sysfunc.go will cause a merge conflict. I will resolve it when that occurs.

Because sysfunc was getting so large, I somewhat arbitrarily broke out the player/enemy functions into their own file. I did not change any of these other functions. If desired, I can revert the change.

[jwalk-511]

Leaving as draft while I gather feedback and generate docs

[jwalk-511]

PR made per request
https://discord.com/channels/845087716541595668/845087716541595671/1352309361740877874
https://discord.com/channels/763583452762734592/851428030094114847/1352492860725006477

Validation of 30% Heal on Burst done by https://discord.com/channels/763583452762734592/851428030094114847/1353185640870842411
Currently in DMs, may be moved to ticket

30% Heal tcl evidence
eburst card triggers furina a1 when furina uses burst. furina a1 cant trigger if furina herself does the healing
Furina only heals for 30% of her health despite having OHC (so HB% stat doesnt do anything)
Burst doesnt proc clam
Kazu heals for well over 30% of his health once a bunch of fanfare is stacked

Conclusions:
30% heal counts as a heal event
Does not count as being emitted by the char that used the elem. burst, and therefore does not trigger ohc/sodp.
Is affected by Incoming HB%, but not by HB%, since there is no triggering char.