Add performance measurements

[novopl]

What?

Add tests to measure performance of various parts of peltak. Especially important is the startup time as this affects the how snappy the auto-completion is.

Some of that could also be exposed to the end users so that they can inspect their setup (every project is different, so better to give tools to handle problems than try to solve all of them ahead).

Why?

One we have the numbers we can start to look into what can be optimized and how much.

How?

Pytest has some plugins for performance tests (pytest-benchmark). We can look into that to see if there's anything we could use.