Cheat framework allowing for custom cheats, sexp for cheat used and On Cheat lua hook

[Kiloku]

I saw #2351 and went a bit overboard. Blame covid, I got bored on sick leave.

This brings a new table type (cheats.tbl and *-cht.tbm). Adding a ship to spawn is optional, as a modder might want to just use the Sexp or lua hook to check if the player used the cheat.

The table looks like this:

#CUSTOM CHEATS

$Cheat:
	+Code: puppyy
	+Message: It followed me home
	+SpawnShip: 
		+Name: Puppy
		+Class: SJ Sathanas
$Cheat:
	+Code: foo
	+Message: bar
	+RequireCheats: YES

#END

If a cheat has RequireCheats as true, then the player needs to enable cheats first, by typing www.freespace2.com (Now that I think about it, maybe I should add the option of using another string for that too...) The table parsing checks if the ship class exists, and pops up a warning if it doesn't, but allows the game to continue.

I had to mess with the original cheat code a bit, the biggest change being that now the Bravos was moved out of that list into the new framework. All original cheat codes still function as usual, as far as I can tell.

[Kiloku]

A quick re-read makes me realize that his needs a few cleanups. Bad variable names, commented out code from previous attempts that should just be removed, probably more. I'll change it into a draft for now.

[Kiloku]

Requested by MediaVPs to add the HLP Bravos!