Introduce a new HtmlVariableListWidget that is similar to
VariableListWidget but for variables with HTML contents.
Update the collectors that use the existing VariableListWidget to use
DebugBarVarDumper to dump the variables using the VarDumper HtmlDumper.
Because many debug bar users may not yet support inline static assets,
default to use the old VariableListWidget for now.
Updated collectors:
* ConfigCollector
* RequestDataCollector
Use the new DebugBarVarDumper class to support dumping variables in
interactive collapsible HTML; the new useHtmlVarDumper() function will
enable this.
Since this will require users to handle the new inline assets provided
by the JavascriptRenderer, make this dumping format optional and default
to the old behavior for now.
