Improve multidimensional array ergonomics

Improve ObjoBasic's support and discoverability for multidimensional data. Today Objo uses arrays of arrays, for example Array(Of Array(Of Integer)) and grid[row][column], which fits the language's generic object model but is harder for Xojo/BASIC users to discover when they try Xojo-style declarations such as Var MyArray(-1,-1).

A first version should make the current model easier to use by documenting 2D arrays/grids, improving the compiler diagnostic for unsupported multidimensional array declarations, and adding a convenient way to build rectangular arrays without row-reference mistakes.