Callstack: Add color-coded icons and better formatting

It would be helpful if the callstack had icons and colors to differentiate between the different types of nodes.

* Color-code events in callstack to match markers e.g. use a green square to indicate paint events
* Need a way to differentiate between website code vs Mozilla code vs OS code
* User actions like "click" should be called out with its own icon
* Function names should be bold or otherwise stronger-looking than its params/superclasses
* Subtle color to ::, <>, () characters

Will make an initial mockup to demo my ideas. Let me know if you have thoughts on what needs to be color-coded.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-663)