feat(developer): add button to show parsed structure

[nknapp]

This is a small extension to the developer.html

• It adds a button "Show structure" that adds a class to the "editor"-div
• Once this happens, additional styles come into effect, that render highlighted spans as blocks with borders and a small label containing the class name.

While this is not perfect (I would rather like to see all modes) this was very helpful when I made that latest changes to the Handlebars language and I would like to see this in the master branch.

[joshgoebel]

Snap of this in action? :)

[nknapp]

[nknapp]

it's more helpful, if you define more classes on youre modes:

[joshgoebel]

Did you try and inline version vs block?

[nknapp]

No, I actually wanted to have a tree view. But it might look better inline, if that doesn't run into layouting problems

I think the optimal solution for developing languages is still another one. Something displaying the test cases in a live reload server. I almost started building that, before I saw developer.html.

I didn't want to spend too much time on tooling, because the actual goal was to write the Handlebars definition.

[joshgoebel]

Add a changelog entry for this improvement?

[nknapp]

Will do.