LQ nash equilibrium

`nnash` from https://github.com/QuantEcon/QuantEcon.jl/blob/13d5b1491f09add09bf8585a5bcf7d7078140e9d/src/lqnash.jl#L61

Used in `markov_perf.md`   and `dyn_stack.md`

Not sure if this is available in other packages, but code is relatively short and could be copied if required and updated to be high performance. Or mabye moved to https://github.com/QuantEcon/GameTheory.jl worst case?