Extension should report config file errors

[mbalc]

if eslint is installed but doesn't work because of a config that's broken - I think the developer should be made aware of that

I just found out today that our config for one of the projects was broken for quite some time

In our specific case:

• we moved eslint config into new flat config format (now it's an eslint.config.mjs)
• I accidentally left one of the plugins in both old (...compat.extends) and new form (ie. eslint.configs.recommended)
• the problem failed silently

I don't know the proper APIs for this but IMO, one of these should happen when such an "InvalidConfigError" error occurs:

1. snackbar error notification

and/or (it'd be pretty cool!)

2. error could be reported as a code problem on the eslint.config.js file, ideally highlighting the place where the config is invalid (that's harder to do, tho)