add _$afterCreateRoot hook to handle new roots in dev

[thetarnav]

Summary

Solid's internal reactivity graph has this problem where roots created with createRoot don't have to be added to its detachedOwner object — it's enough that they are able to get to the context walking upwards.
But not for devtools, all the createRoot calls are effectively invisible for a debugger that wants to walk downwards to analyze the tree. And roots are very common — components such as <For>, <Index> and <Suspense>; primitives that want to control disposal like createLazyMemo; and even render function that is creating the main branch of the reactive tree.

Having a special hook that would fire with every newly created root would let us handle all of those cases.

Then it could be used easily for tracking:

import { registerRoot } from "solid-devtools";
window._$afterCreateRoot = (root) => {
   registerRoot(root);
}

Related discussion: #860
And devtools issue: thetarnav/solid-devtools#15

How did you test this change?

Added a new test case to dev.spec.ts

[coveralls]

Pull Request Test Coverage Report for Build 2534785309

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 88.99%

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Totals
Change from base Build 2482635498:	-0.02%
Covered Lines:	1254
Relevant Lines:	1337

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💛 - Coveralls

[ryansolid]

Thanks.