Allow ternary to reference marker fields in marker labels

[carverdamien]

While migrating DebugBreakMarker, I ran into the need to do this {marker.data.msg ? marker.data.msg : marker.data.expr} where msg and expr are both strings. We basically want to display msg unless it's empty or null, then we display expr.

https://bugzilla.mozilla.org/show_bug.cgi?id=2023960
https://phabricator.services.mozilla.com/D288218

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