See discussion #219

This PR adds the preact debug tools to the runtime. Webpack now creates two versions of the runtime. A runtime.js including the debugtools and a runtime.min.js without them. Bundle size is nearly the same.

If SCRIPT_DEBUG is set to true, the plugin enqueues the version with debug tools. Both versions are minified.
To make it work I had to extend the webpack config and return multiple configs - that's why common config is stored in a shared config. Also the DefinePlugin is used to build a constant which can be checked in the runtime on whether to import the debug tools or note. preact/debug cannot be externalized and needs to be the first import, that's why a flag is used.

I'm not a webpack expert, so if there's a better way to do it let me know.

How to test:

1. Install the preact devtools
2. Run npm build
3. In .wp-env.json set config.SCRIPT_DEBUG to true
4. Start the local dev environment via wp-env start.
5. Enable "Client side navigation" in the plugins settings
6. Load the frontpage and check that runtime.js was loaded. Preact devtools should be enabled (and not show a warning about missing renderings). Preact devtools should show some directives.
7. In .wp-env.json set config.SCRIPT_DEBUG to false
8. 4. Start the local dev environment via wp-env start.
9. Load the frontpage and check that runtime.min.js was loaded. Preact devtools should not be enabled/active or show a warning about missing renderings.
10. All other things should work as expected (eg client side navigation).