nix: skip check completely when checkConfig=false

[ian-h-chamberlain]

Fixes #5753

Description

Also add an allowUnknownSettings option to enable checking everything except for unknown settings.

Checklist

• Change is backwards compatible.

  NOTE: the caveat here is that users who previously had checkConfig = false will now have less checks performed than they did previously, but frankly I'm not sure it was even checking anything useful, since "unknown settings" seem to be emitted much later vs other kind of errors. A config like this only errors on keep-outputs and doesn't complain about the unknown setting at all:

   keep-outputs = 123
   some-other-setting = true

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.
  NOTE: just testing the new tests I added and other nix.conf related tests

• Test cases updated/added. See example.

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  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

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