types: Rework TFunction and translation key suggestions

[kkuegler]

Allow 'other-ns:some.key' syntax regardless of current namespace(s). However, we no longer provide these keys via TS autosuggestion, to not flood the suggestion box with these.
This uses the 'DisableSuggestion' helper type, which unfortunately allows any capital case prefix to the actual 'other-ns:' prefix.

For getFixedT with multiple namespaces, all the keys in these namespaces are now suggested without 'ns:' prefix. Using such a prefix is still possible, though.

Technically, we treat cases with a fixed single namespace and multiple namespaces the same in TFuncKey by flattening an array of namespaces into a union of namespace strings. Using this we can use distributive conditional types to handle each namespace individually. See https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types

For NormalizeReturn we use a new DistributiveKeyOf<T> helper. This finds keys that are available in only one/some of the namespaces, whereas 'keyof T' would only find them if they were in all of the namespaces.

We simplify and rework the "with options" overload of TFunction. This no longer allows passing the namespace. That use case is already covered by other overloads before that. This overload was the reason for erroneously suggesting unprefixed keys for not currently active namespaces.

The probably helps with #1887, too.
fixes #1889

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

[coveralls]

Coverage: 91.95%. Remained the same when pulling 18c6274 on kkuegler:ns-prefix-1889 into 5bd9ac0 on i18next:master.

[stale]

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