Implement Slicing Arrays by ID

Implements the following syntaxes:

let a = [ 1 2 3 ]
echo @A[0]
echo @A[1]
echo @A[2]

echo @[echo 1 2 3][0]
echo @[echo 1 2 3][1]
echo @[echo 1 2 3][2]

echo [1 2 3][0]
echo [1 2 3][1]
echo [1 2 3][2]

Next up: array range slicing