Provide DTD URI to the Analysis Server

[elliette]

As part of supporting the Dart Tooling Daemon integration, IDEs need to provide the DTD URI to the Analysis Server.

Here is the commit passing the DTD URI to the Analysis Server in VS Code: commit

We will need the Dart Plugin to do the same.

[elliette]

Documentation for dart/connectToDtd: https://github.com/dart-lang/sdk/blob/bc9def5da7804243a9cce531e3512ab94a569fcd/pkg/analysis_server/tool/lsp_spec/README.md?plain=1#L280