Migrate mps to extension

[JamieMair]

Summary:

• Moved all MPS code into an extension to allow the module to be loaded with/without MPS support. This is mainly to fix the automated unit testing as MatrixProductStates.jl is not a registered package. See this discourse post to learn more about extensions.
• Switched using Plots.jl to using CairoMakie instead to have one plotting package in the examples project.
• Removed all unnecessary deps from the main Project.toml
• Merged in changes from the latest main branch
• Moved MPS tests into the MPSExt folder (they will still be discovered)

It would be helpful to check whether the code in your test scripts still works as expected as I have only testing the plotting part of the code.

@AdamSmith-physics would you also be able to look at the deps in your Project.toml in MatrixProductStates.jl? I suggest moving all the test deps into the [extras] tab, and removing packages like Revise and BenchmarkTools (unless used in the code) and putting them in your global environment.

[AdamSmith-physics]

I am happy with your changes. Please go ahead and merge!

[AdamSmith-physics]

Note I might also move some of this functionality into the MatrixProductStates.jl package. I can clean this up when I get round to doing it.