The dart layer requires the official LSP server as the
emacs mode alternative is not longer properly maintained.
Therefore I have added some more description of how to
install the server properly and let the layer load
the LSP layer automatically.
In addition I have removed the unsupported variable
`dart-server-enable-analysis-server` from the
documentation.
See https://github.com/bradyt/dart-mode/wiki/LSP for
details.
