Provide helpers for convenient array initialization

[PaulRBerg]

As suggested by @andreivladbrg privately, it would be helpful if PBRMath came with solarray-style functions for convenient array initialization:

UD60x18s[] memory nums = Solarray.UD60x18s(ud(1), ud(2), ud(3));

Here are two ways in which this feature could be implemented:

1. Provide a PRBMathSolarray library in the test/utils directory (libraries cannot be extended, unfortunately)
2. Provide a new helper in each value type's Helpers.sol file, i.e. a join helper with multiple overloads

The first solution is more verbose, but clearer, whereas the second solution is more elegant (in a programming sense). Still, it could be misunderstood to mean "joining" numbers in a mathematical sense. TBD.

Side note: I happen to have already added these helpers for PRBMath in my fork of Solarray