Allow customisation of the root error boundary

[infomiho]

React Router v6 support specifying a React component that will be rendered in case a page fails to render (for which ever reason - invalid data, developer syntax error etc.).

In #2256 we introduced a simple default root error boundary. We want to allow users to customize this component: something like we have for the client.rootComponent.

[Martinsos]

Related to #1243 .

Oh ok so React Router v6 PR introduces a default error boundary, nice!

One easy way for them to provide their own error boundary is just to set one up in the rootComponent, it will be more specific then the default one we defined, and they can already do taht out of the box. But I understand that errors might happen on the higher level, during routing, and then it is important they can customioze to top level default one? Ok, makes sense.