feat: add support for config export

[eduardoboucas]

Adds support for in-source configuration in edge functions. Rather than forcing people to have a netlify.toml where they declare the paths for each edge function as well as other configuration parameters, we allow them to declare the same parameters from within a config export in the function's entry point file.

It works by looking for a config export, which should be a function. When one is found, Edge Bundler will invoke that function at build time and use its output (which must be serialisable to JSON) as the input for the configuration object.

Here's an example:

netlify/edge-functions/hello.ts

import { Config, Context } from "https://edge.netlify.com"

export default async (req: Request, context: Context) => {
  return new Response(`Hello, ${context.geo.country.name}`)
}

export const config: Config = () => ({
  path: "/hello"
})

NOTE: Edge functions declared using this format will run after any edge functions declared in the TOML, and they will run in alphabetical order (within the group of functions defined using this method). So if you're chaining multiple edge functions and you want to control the order in which they run, the TOML declaration method is preferred.

[eduardoboucas]

We're no longer creating multiple bundles, so I've simplified this a bit.

[jackiewmacharia]

Looks good!