Autodetect pre-commit executable on more platforms / with more installation methods

[lorenzwalthert]

Any installation method that puts the executable on $PATH will be detected anyways. We can further look in more places so people don't have to link manually. The hierarchy should be:

• detect on PATH.
• if not there, detect with conda (via reticulate).
• if not there, detect os specific via pip and brew.

We aim to detect installation methods that are officially supported.

Status quo

platform	conda (env reticulate)	pip (user)	brew
macOS	✅ via reticulate	✅ ~/Library/Python/:py-version/bin	✅ /usr/local/bin
linux	✅ via reticulate	✅ ~/.local/bin	❌ not available
windows	✅ via reticulate	✅ ~\AppData\Roaming\Python\Scripts	❌ not available

Legend

❌: not implemented
✅: implemented
⚠️: WIP

Detecting on a platform should be tested on CI/CD similar to existing test the ones introduced for macOS and pip in Linux in #121.