Add support for LocalDev.elm modules

[pithub]

This lets you split the code for LocalDev.elm into multiple modules, which can be useful to add multiple "plugins" like the REPL or program-test.

All modules in extra/LocalDev/LocalDev will be installed automatically in the user project's elm-stuff/lamdera folder and can therefore be imported in extra/LocalDev/LocalDev.elm.

When developing new versions of LocalDev.elm or its modules, all changes will be detected automatically and will override the standard versions in elm-stuff/lamdera, so the new versions can be tested without having to re-build the compiler.

[pithub]

Commit 5f9244f splits the code in extra/LocalDev into two sub-directories: runtime-src and tooling-src (see here). Quote from the readme.md:

The runtime-src/LocalDev.elm file is what gets paged in as the entry point for lamdera live. It may import other files from the runtime-src directory.

The files in tooling-src are not used in the final compiler build, they are only here to assist editor tooling when developing runtime-src/LocalDev.elm.

At runtime, the whole subtree under runtime-src is copied into the local project's elm-stuff/lamdera directory. This allows for example to add vendored packages like elm-composer.

[supermario]

Looking good – will test shortly.

We should also update the notes in https://github.com/lamdera/compiler/blob/lamdera-next/extra/readme.md#livejs