Add a summary of time spent in different phases of a Darker run (in debug output)

[akaihola]

To help troubleshoot and fix performance issues, it would be helpful to be able to get a summary of time spent in different phases of a Darker run. The summary could be output when debug mode is enabled with --verbose.

If there's a good package in PyPI to make this easy, we could add it as an optional dependency, and in case it's not installed, --verbose could instruct the user to pip install it to get a timings summary.

Steps:

• Research existing Python packages for measuring and summarizing timings
• Design the granularity: which phases of a Darker run should be measured?
• Implement and create a PR

Related:

• Document, profile and analyze current performance issues #26
• No darkerd HTTP API server similar to blackd #59
• Support ruff format as a formatter #521