Undusts spell-code

[ShiftyRail]

Foreword

Spellcode is quite advanced and nice to work with. It's also very old, undocumented, non-standardised, and has a lot of confusing var names that I think make it a bit harder for a new guy to know what's going on.

What this does

This adds comments to spell.dm explaining what the vars do.
This changes the defines in spellcode from Sp_X into SP_X so that VSCode syntax highlighting works better on them.
This changes all instances of duration, cooldown_min, charge_cooldown_max to use the SECONDS define for standardisation and to know at a glance what a spell cooldown is without unnecessary mental arithmetic.
This renames the var charge_max into charge_cooldown_max for clarity. When the var is about a charge counter (or anything else than seconds), the define CHARGES is used to signify the reader that the spell uses a fixed number of charges.

The define is:

#define CHARGES *1

So essentially this does nothing.

Why it's good

Keep the codebase a little neater and makes comparing spells a bit easier. There's nothing player-facing in this PR.

How it was tested

I compiled and I checked that I indeed only changed vars and defined for spells. I did not touch the mech spells because I didn't fully understand what they were doing.

Changelog

Not necessary as this is not player-facing.

[ShiftyRail]

I had forgotten a file.