nixd: diagnostic control

[inclyc]

Add a configuration field to allow users overriding which kind of diagnostic should be emitted.

As mentioned in #475 , we may allow users to toggle each diagnostic. If they like/dislike it.

[inclyc]

Proposed change: I would like to add a new configuration option under diagnostics, maybe suppress is a suitable name.

{
   "diagnostic": {
    // Array of strings, specify diagnostic IDs need to be suppressed.
    "suppress": [ ]
  }
}

For better testing there might also be a CLI flag, for better testing.