Feature: Allow files to be highlighted based on highest severity

[mtrajano]

This would allow the user to see a high level overview of which files have errors, warnings, etc.. similar to how vscode highlights the tab with the worst severity. This is useful if you have a lot of files open and you have them folded by default. Let me know if this sounds interesting and I can get a pr out for it.

I got it working by making some small changes to providers.group to have file attributes, instead of just returning an array of diagnostics for each file. This allows for some other interesting possibilities: sort by files with the worst severity, seprate the count by severity etc.. Example of how it can look: