cmd/trace: add almost full support for v2 traces in the trace viewer

cmd/trace: add almost full support for v2 traces in the trace viewer

This change refactors the cmd/trace package and adds most of the support
for v2 traces.

The following features of note are missing in this CL and will be
implemented in follow-up CLs:
- The focustask filter for the trace viewer
- The taskid filter for the trace viewer
- The goid filter for the trace viewer
- Pprof profiles
- The MMU graph
- The goroutine analysis pages
- The task analysis pages
- The region analysis pages

This CL makes one notable change to the trace CLI: it makes the -d flag
accept an integer to set the debug mode. For old traces -d != 0 works
just like -d. For new traces -d=1 means the high-level events and -d=2
means the low-level events.

Thanks to Felix Geisendörfer (felix.geisendoerfer@datadoghq.com) for
doing a lot of work on this CL; I picked this up from him and got a
massive headstart as a result.

For #60773.
For #63960.

Change-Id: I3626e22473227c5980134a85f1bb6a845f567b1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/542218
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>