support Client Workspace Configuration in Dart LSP

[flo80]

Check for existing issues

• [X] Completed

Describe the feature

With the merge of #7220 Dart LSP is working but not all features of the LSP are supported.

Specifically the Client Workspace Configuration is missing.

It would be great if this options could be configured in setting (like in the VS code extension) and used by the LSP (to have consistent behavior e.g. when formatting).

As far as I can see, the LSP implementation would allow this, e.g. the YAML LSP provides some values.

If applicable, add mockups / screenshots to help present your vision of the feature

Easiest is probably to add the LSP workspace config in global/local settings like in VS code

{
  "Dart": {
    "dart.lineLength": 200
  }
}