feature: toggles/flags rework

[folke]

Did you check the docs?

• I have read all the snacks.nvim docs

Is your feature request related to a problem? Please describe.

Currently we have toggles for ignored, hidden and follow.

• they're mapped globally for every source.
• show in the title as a flag
• have an associated action

Describe the solution you'd like

toggles/flags should be more generic and be able to be configured per source.

Like for grep opts.toggle = {...}.

Describe alternatives you've considered

...

Additional context

No response

[maskudo]

An option for toggling between diagnostics based on severity would be nice. I noticed there is such toggle in trouble already.