Debugger pretty-printing icon can be confused with the Style Editor icon

[fvsch]

Because, well, it's virtually the same { } icon, as pointed out by @mcroud in #36.

We could explore other options for this icon, and perhaps other options for surfacing the "Pretty print" and "Blackbox source" actions (e.g. improved icons, text labels, right-click menu entries for the tab and in the Sources pane).

Edit: the context menu issues are addressed by https://github.com/devtools-html/debugger.html/issues/7497, so we should limit this issue to just "how do we make the Pretty Print button clearer": new icon, maybe showing a label with the icon? If we can make this work well for the "Blackbox source" button too, that's a nice bonus.

[fvsch]

Worth noting that Chrome's debugger uses the same kind of { } icon, so it may be worth keeping it if we don't have a clearly better solution.