* Update dart analysis_server command
In 2021 the dart team added a new sub-command `language-server` to
replace the original `./snapshots/analysis_server.dart.snapshot --lsp`
convention for starting the language server.
c224cc2e0d
* Add ale_dart_analysis_server_enable_language_server option
This allows users to opt-in to the new `dart language-server` command.
* Enable ale_dart_analysis_server_enable_language_server option by default
* Update doc/ale-dart.txt
Include the dart version number where the `dart language-server` command
was added.
