Move environment variable files to src/server and src/client

[sodic]

After merging #753, we've separated the server code from the client code. However, the files .env.server and .env.client are still in the project's root directory.

We should move them to src/server and src/client respectively. Also, it might make sense to reconsider the name src, as it can contain more than just source files (e.g., images, .env files, documents...).

While we're at it, let's also make sure to warn users if they put them in the wrong place (as we currently do for .env).

[sodic]

This issue is no longer relevant with changes introduced released in Wasp 0.12.0 (merged in #1686 )

Server code and client code are mixed under src and all config files now reside at the top-level, as is the case with most popular frameworks.