title: How do I set indenting options for a language in a VSCode extension?
body: <p>I'm working on a VS Code extension for Dart. The convention for Dart is to indent with 2 spaces (ugh, I hate this too) so I'd like to automatically set it when the user opens a Dart file rather than use their defaults.</p>
  
  <p>There's an <code>insertSpaces</code> property in the <code>FormattingOptions</code> class but it's not clear how I can set this, nor the best way to do it (it would be nicer to set this at a language level than to keep setting it whenever a document is opened, for example).</p>