add Elm support to webpack encore

[heithemmoumni]

No description provided.

[stof]

Is elm-webpack-loader actively maintained ? Last commit was in October 2019 and it depends on vulnerable old versions of its dependencies, which is a bad signal to me.
Encore already started the work to migrate to webpack 5 (see #645) to be ready at the time it is released. If this loader is not maintained anymore, it won't be upgraded to be compatible with webpack 5, and Encore will need to revert the feature at the time of the migration.

[stof]

@heithemmoumni any answer to my concerns about the maintenance of the elm-webpack-loader ?

[heithemmoumni]

i already open a pull request to update all deprecated dependencies and support node 14