- eglot-jl.el does not have to find the project itself (this could be extended
  to simplify `eglot-jl--project-try` as well)
- `eglot-jl-default-environment` becomes unnecessary
- the default environment is automatically adapted to the julia version in use

This also simplifies running the language server manually, since no argument is
mandatory anymore.

`eglot-jl.jl` takes a source directory as first argument. It reuses internal
functions from `Base` to find the correct project for this directory.

The major version number is bumped following the removal of the
`eglot-jl-default-environment` customization option.