Add repository custom instructions for GitHub Copilot

[ianna]

Create uproot repository custom instructions files that give Copilot additional context on how to understand uproot project and how to build, test and validate its changes.

Location:

.github/instructions?filename=*.instructions.md

Please see https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions for details