Add keywords for scoreboard player operations

[MysteryBlokHed]

If you want to perform a scoreboard operation on two databind variables, you'd currently have to do it like this:

scoreboard players operation --databind var1 = --databind var2

It would be easier to have a keyword that becomes --databind var1 or --databind var2. The :tvar keyword almost does this, but it also has score at the beginning (eg. :tvar var1 -> score --databind var1). Adding a new keyword like :gvar to get just what you'd need to perform an operation would be a good idea. That way, the above could be written as such:

scoreboard players operation :gvar var1 = :gvar var2

Adding another keyword like :varop or something that becomes scoreboard players operation would shorten the above even further to the following:

:varop :gvar var1 = :gvar var2

[MysteryBlokHed]

Mixing multiple Databind keywords in the way the aforementioned example does could be hard to implement in the tokenize() function. Adding something to handle :varop in preprocess.rs would most likely be easiest.