Performance inspection tools

Right now, Pkl does not offer any tools for debugging the performance of a Pkl program.

There should be tools to help inspect perf issues.

Tools like flame graphs and heap dumps would be quite useful.

The issue names no files, tests, or entry points. Begin by surveying Pkl's existing debugging and performance tooling, then define the supported inspection workflows for flame graphs and heap dumps. Done should include agreed tools and a documented way to use them for investigating Pkl program performance.

Written by the indexing model from the issue text.