Add nfc_normalize_idents flag

[Jules-Bertholet]

Addresses #6058 @ytmimi

[ytmimi]

@Jules-Bertholet when you have a moment can you remove the fix for #6069 from this PR and open a separate PR that independently addresses that issue.

[Jules-Bertholet]

The test for the nfc_normalize_idents relies on the #6069 fix to pass (it checks that NFC-normalizing idents in trait declaration works, which relies on rustfmt not immediately crashing in that situation.) So I can split it out, but this PR will then be blocked on that one.

[ytmimi]

The test for the nfc_normalize_idents relies on the #6069 fix to pass (it checks that NFC-normalizing idents in trait declaration works, which relies on rustfmt not immediately crashing in that situation.) So I can split it out, but this PR will then be blocked on that one.

Yeah, I'd rather split them out. The fix for #6069 will be easier to land, and then we can keep iterating on this new configuration.