feat: support arrays and array indexes/slices

[FyreByrd]

Closes #26.
Part 1/4 of #35.

let arr = [1, 2, 3]; // array literal
let x = arr[0]; // array index
let slice = arr[:]; // slice of the whole array
let fullSlice = arr[1:2:-1]; // full array slice
let y = arr[]; // index with no expression. Not sure on this. artifact of parser rules
let arr2: i32[] = [1, 2, 3]; // explicit type

Potential issue:
I ran into a common lookahead prefix error when trying to require an expression in between the brackets for array indexing, but make it optional for array slices (i.e. arr[:]).
The best solution I could find was to just allow for empty brackets for array indexing. I'm not a big fan of it. I wish I could make it a syntax error, but so far I think the best I can do is a semantic error.