There should be a hook exposing jsTree to addons

[rondale-sc]

In glimmer-app.ts there are a few addonProcessTree calls that expose things like:

• postprocessTree
• preprocessTree

For the src and css trees. However, since the jsTree is from the result of the src tree going through the TS compiler inside the javascriptTree function (see: https://github.com/glimmerjs/glimmer-application-pipeline/blob/master/lib/broccoli/glimmer-app.ts#L407) there is no opportunity to inject non typescript modules into the JS output.

This was made apparent to me when I tried to create a module that was in the jsTree before rollup happened in the package method. I was forced to modify the srcTree and add a TS file there which was inconvenient. I think on L407 (linked above) we should add a addonProcessTree to expose the javascriptTree to user space.

Any thoughts on this?