Enable the use of HiGHS as an open-source solver

In `README.md` you observe that "Any solver compatible with [JuMP](https://jump.dev/) can be used, although performance with open-source solvers (e.g. Clp, GLPK) may be significantly slower than with commercial solvers."

Are you aware of our MIT-licensed software [HiGHS](https://highs.dev/) and, in particular, the experiences of the PyPSA team who observed that it's vastly better than Clp and GLPK on their energy system models. Indeed, HiGHS is competitive with Gurobi on many of their models. The comparison with Clp and GLPK is [general](http://plato.asu.edu/ftp/lpbar.html).

More details of the PyPSA experience are available from [here](https://highs.dev/assets/HiGHS_funding_proposal.pdf).

Since HiGHS is available via JuMP - indeed, it's used as the example solver in JuMP's documentation - it should be trivial to use it rather than Clp or GLPK.