Make various GC markers schema-based

[mstange]

Markers with types GCSlice, GCMinor and GCMajor currently have custom code for their tooltips. Their fields timings / nursery / nurseryTimings are not declared in a schema.

We should add marker field formats for these JSON objects and declare the fields in the schema, for #5302.

So then the custom tooltip code would still be there, but it would just be called for a field whose format is e.g. gc-minor-nursery-timings, rather than for a marker whose data.type is GCMinor.

┆Issue is synchronized with this Jira Task