[Fiber] Clear existing text content before inserting children

[acdlite]

Fixes a bug when updating from a single text child (or dangerouslySetInnerHTML) to regular children, where the previous text content never gets deleted.

Based on #8304 (compare view).

[acdlite]

Here's a description of what I've added:

• In reconciliation phase, check if the host component is going from a single text child to normal children. If so, schedule a ContentReset effect. The logic for checking if the host component should render with a text child is now part of the host config. We can't just check for text and number children because of special cases like dangerouslySetInnerHTML.
• Whenever a node is being inserted, check if the parent has a ContentReset effect. If so, reset the text content of the parent and remove the flag from the effect tag. Resetting the text content of a host instance was also added to the host config.
• In the commit phase, perform any ContentReset effects that weren't already handled by child insertions.

[syranide]

If we're concerned with performance then IIRC setting textContent or innerHTML is unintuitively significantly slower than simply clearing the children one by one.

[acdlite]

This only gets called when there's a single text child

[sebmarkbage]

That's not true because dangerouslySetInnerHTML can have multiple children.

[acdlite]

Oh right that's true

[sebmarkbage]

Ok, accepting this but not the things it build on top of. So this will either have to wait or rebase on top of master.

[sebmarkbage]

Nit: This should've been cached in a local variable above instead of using the object form.

[arcanis]

Just a small feedback after implementing shouldSetTextContent in a custom renderer: it could be useful for the hook to also get the type of the parent instance. It would enable this kind of logic, where we want to ensure that text nodes are wrapped into a specific type, but also want to allow using this type directly in case the user wants to add extra props to the component:

createInstance(type, props) {
    return new (components.get(type))(props);
}

createTextInstance(text) {
    return new (components.get('text'))({ text });
}

shouldSetTextContent(type, props) {
    return type === 'text';
}