Make pn.state._is_pyodide public

[MarcSkovMadsen]

When creating applications that run on pyodide and especially when creating app code that should be able to run on both server and in pyodide it would be really helpful to know which environment you are in.

Right now I have to look this up every time I create apps for dual environments. Why not just make pn.state._is_pyodide public? I.e. rename to pn.state.is_pyodide.

[x] Yes. I would be willing to implement the change if I'm told this is ok.

[philippjfr]

I have no real objection to this, except that it seems like it shouldn't be Panel's job to detect this for you. That said, it clearly is quite helpful.