[svsim] Trace options for the Verilator backend

[kammoh]

• Adds the following options for generating traces using the Verilator backend:
  • maxArraySize: Maximum array depth for tracing
  • maxWidth: Maximum bit width for tracing
  • traceDepth: Depth of tracing

The corresponding Verilator flags are only added if the above integer values are > 0.

• Adds TraceStyle.Fst for generating FST traces.

• Removes the unused filename field from TraceStyle which seemed very confusing. The trace file name (stem) is only configured through CommonSimulationSettings.traceFileStem.

• svsim: Try to figure out the project root directory in the presense of build-system sandboxing (e.g., as in the newer versions of mill). Introduces workingDirectory to be used instead of workingDirectoryPath. I've had this issue with my own Chisel projects, but decided to sneak the fix in as it was causing problems while testing this PR.

I guess, technically an API change due to the change of the TraceStyle.Vcd case class?

Replaces #4910

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[kammoh]

One question is if the default values for maxArraySize and maxWidth should be non-zero. Not being able to see arrays or wide signals in the waveform could be confusing to new-commers (or people less familiar with Verilator). I don't think performance is major concern as waveforms are only used for debugging, and from my experience any extra overhead is unnoticable. If the answer is yes, what choice values sounds reasonable?

[jackkoenig]

This looks like a better API to use generally should we deprecate val workingDirectoryPath and suggest people use this?

[kammoh]

I can't think why not, so I moved getProjectRootOrCwd to the Simulation object so it's accessable elsewhere (and public).
I also added CHISEL_PROJECT_ROOT env variable as an alternative way of specifying/detecting the project root. Then I used it for buildDir of scalatest.TestingDirectory.
Now scalatest.ChiselSim build directory in my mill projects are created in the workspace folder (./build) instead of being hidden under ./out/module_name/tests/testOnly.dest/sandbox/build.

[kammoh]

@jackkoenig @seldridge this is now ready for your reviews.
The getProjectRootOrCwd change should have ideally been on a separate PR, but it was needed for testing and I think it should be all fine.