feat!: integrate router with app and app.fetch

[pi0]

This is a major upgrade for h3! Since the initial version, h3's core was modeled as a stack runner, using an array of middleware (path prefixes and event handlers) matching them one by one, and stopping as soon as the first middleware responded.

We then added a new router system to h3 using radix3 which uses route pattern matching.

With advancements of rou3, we can actually have a much smarter combined app.

In order to keep compatibility with previous versions, the h3 app smartly chooses between 3 strategies (in order):

1. global middleware without prefix registered with app.use(handler)
2. middleware with route registered with app.use("/prefix", {}) (this is not encouraged due to perf overhead!)
3. routes with method registered with app.all(handler or app.[method](handler)

Any of the middleware can still return a response to stop the process (this is to keep more backward compatibility) but there are new behavior changes:

• middleware registered with app.use("/path", handler) only match /path route. For matching all subpaths like before, it should be updated to app.use("/path/**", handler)
• The event.path received in each handler will have a full path without omitting the prefixes. useBase(base, handler) wrapper can be used if old behavior is desired.
• custom match function for app.use is not supported anymore (middleware can skip themselves based on path)

Other changers:

• app.options (to access config) is renamed to app.config (app.options registers an event handler for OPTIONS HTTP method)

Migration guide: #787