Instantiable env

[developit]

This PR builds on #24. In #24, DOM classes/prototypes became shared across all Documents created via undom():

importundomfrom'undom';constdoc1=undom();constdoc2=undom();doc1.defaultView.Node===doc1.defaultView.Node// true

With this PR, undom() continues to return shared constructors. However, a new undom.env() method returns a new factory function (akin to undom()) that can be called to create documents with a new set of constructors:

importundomfrom'undom';// as with #24, documents default to the shared environment:constdoc1=undom();constdoc2=undom();doc1.defaultView.Node===doc1.defaultView.Node;// but now we have an escape hatch:constenv=undom.env();constdoc3=env();doc3.defaultView.Node===doc1.defaultView.Node;// FALSE!

This means we still encourage using the shared globals, but allow creating a new set of them if necessary.

It also opens up an interesting option for plugins, which were proposed a while ago, but would have been difficult to implement.

importundomfrom'undom';importserializationfrom'undom/serialization';importshadowfrom'undom/shadow';functionapplyPlugin(dom,plugin){plugin(typeofdom==='function' ? dom() : dom);}// create a clean environmentconstenv=undom.env();applyPlugin(env,serialization);applyPlugin(env,shadow);// now all documents created for the environment have those plugins:constdoc1=env();constdoc2=env();// fun part - this works with the default environment too:applyPlugin(undom,serialization);constdoc3=undom();// has the plugin

[bmeurer]

Very nice!