Compound assignment operators (e.g. "+=")

[Joy-less]

Adds support for: +=, -=, *=, /=, %=, ^=, and ..=.

While these operators are not present in Lua, they are present in Luau and save a lot of time.

Example:

local a = 0
a += 5
print(a)

Converts to:

local a = 0
a = a + 5
print(a)