Implement workspace_configuration for Dart LSP

[flo80]

This PR addressed https://github.com/zed-industries/zed/issues/8558 and allows the Dart Client Workspace Configuration.

Differing from the issue, the settings are used from the LSP settings section, example:

{
 "lsp": {
    "dart": {
      "settings": {
        "lineLength": 200
      }
    }
  }
}

Note: this only works for the global settings (not folder specific)

Release Notes:

• Fixed missing client workspace configuration of Dart LSP (#8858).

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[flo80]

@cla-bot check

[cla-bot[bot]]

The cla-bot has been summoned, and re-checked this pull request!

[mrnugget]

Thank you! Updated the release notes a bit and will merge once CI is green.