Send DTD and DevTools URI to widget preview

[helin24]

Follow up to #8595

At the moment I don't see consequences to having or not having the DTD connection, but I expect that in the future, the widget preview will respond to theme change updates over DTD. (and potentially other things?)

Based on the conversation in #8595 (comment), the DevTools URI also does not have a visible consequence, but it should prevent flutter tools from starting its own DevTools instance when widget-preview is run.

[bkonyi]

At the moment I don't see consequences to having or not having the DTD connection, but I expect that in the future, the widget preview will respond to theme change updates over DTD. (and potentially other things?)

If not given a DTD URI for the DTD instance hosted by the IDE, the widget previewer won't be able to respond to editor events (e.g., changing the set of previews displayed based on the currently selected file, responding to theme change events, etc), which would significantly reduce the UX when using the previewer embedded in the IDE.

[helin24]

@bkonyi and I discussed a little further and decided I'll try to implement editor service RPCs in the plugin:

• navigateToCode
• getActiveLocation

And the events:

• activeLocationChanged
• themeChanged (this is lower priority)

They'll be in upcoming PRs if easy to do. Otherwise we'll decide where to prioritize.