feat: execution trace gantt chart

[viktorasm]

When debugging highly parallelized task executions, it's not immediately obvious where the bottlenecks are.

This PR allows dumping timings of the execution as Gantt chart (Mermaid syntax). Here's a sample of of a real-word invocation:

Having this makes it so much easier to understand, how well things parallelize, are tasks unnecessarily blocked, what are the major bottlenecks, etc.

Usage:

• if user provides --execution-trace-output flag, then task invocation writes contents of the gantt chart to it.