v2021-03-15

Support projects with `elm.json` not at the root

Expand the Elm Editor to support compiling modules for which we want to use an `elm.json` file that is not at the root of the project file tree:

+ Update methods in Elm Fullstack for more consistent APIs when modeling file paths: Use sequence of strings instead of depending on OS-specific separator character.
+ Expand the `ExecuteFileWithArguments` method in Elm Fullstack to support a working directory different from the root directory.
+ Expand the Elm Editor backend to forward a working directory for the `Elm make` command as given by the frontend.
+ Expand the Elm Editor frontend to select the matching `elm.json` file for the Elm make entry module chosen by the user.