[vm] Improvements for --trace-debugger-stacktrace output.

Improves the readability of the tracing output by adding beginning and
ending markers for debugger stacktrace collection and using specific
spacing instead of \t in output, so that output is more naturally nested
via indention:
* beginning/ending markers are not indented,
* frames are indented by two spaces, and
* additional output while collecting a frame is indented by four spaces.

Adds tracing for DebuggerStackTrace::CollectAsyncAwaiters(). A different
ending marker is used in the case when the collected trace is discarded
due to a lack of async awaiters.

ActivationFrame::GetSavedCurrentContext now takes an optional out
parameter for the current context variable index, which is only used by
CollectDartFrame and used to print the index there. This eliminates
extra output when this method is used outside collecting stack traces.

When the current context variable index is requested,
GetSavedCurrentContext ignores the cached context if any so that the
index is appropriately set if an appropriate variable is found.

By default, skips tracing of non-collected frames and repeated async
suspensions in async awaiter traces. Use the new
--trace-debugger-stacktrace-verbose flag to add tracing for these cases.

TEST=ci (tested manually, only affects debugging output)

Change-Id: I2978d540f41b444251a61b62a57b6785a3dd8df0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467300
Auto-Submit: Tess Strickland <sstrickl@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>