feat: implement standalone fromContext

[divmain]

It might be worth looking into how we can consolidate this new fromContext implementation with the state manager's internal fromContext implementation. This new one is much simpler, since it only cares about one variety of context.

Once merged, you will be able to do things like this:

lightning-labs/example/src/modules/x/contextChildDetached/contextChildDetached.js

Lines 2 to 12 in 651e58f

	import { fromContext } from '@lwc/state';
	import childStateFactory from 'x/childState';
	import parentStateFactory from 'x/parentState';
	export default class ContextChild extends ContextfulLightningElement {
	parentState = fromContext(parentStateFactory);
	get nameProvidedByParent() {
	return this.parentState.value?.name ?? 'not available';
	}
	}

[rax-it]

I thought a bit about separating a base-context signal so that it can be reused in SM and here but I agree with you that this is not much bloat. LMK when this is merged, I can add the disconnectContext stuff as it's already merged now.