Package setup

[mtfishman]

This makes the package installable and also adds CI, a docs template, etc.

It also demonstrates the output of ITensorPkgSkeleton.jl for an existing package. This PR is basically just the output of executing:

julia> using ITensorPkgSkeleton: ITensorPkgSkeleton

julia> ITensorPkgSkeleton.generate("LabelledNumbers"; ignore_templates=["src", "test"])

and also editing some test code to remove references to NDTensors, adding dependencies to the Project.toml generated by ITensorPkgSkeleton.generate, etc.

To-do (maybe left for a future PR):

• Move code that depends on BlockArrays.jl to a proper package extension. (Tracking in Move code that depends on BlockArrays.jl to a package extension #2.)
• Use ITensor organization Github Action workflows, once they are made available in ITensorPkgSkeleton.jl through Update template to use organization actions ITensorPkgSkeleton.jl#11. (Tracking in Use ITensor organization Github Action workflows #3.)
• Create examples and docs. (Tracking in Create examples and docs #4.)