Suggest dropping incognito: "split" when present in manifest.json

[Rob--W]

See https://bugzilla.mozilla.org/show_bug.cgi?id=1876924 for context.

incognito: "split" is not supported in Firefox, only in Chrome.

In Firefox, incognito: "spanning" is the only supported value that enables users to opt in to allowing extensions access in private browsing mode. The only other supported value is incognito: "not_allowed", which prevents users from using an extension in private browsing mode.

In bug 1876924, I'm patching Firefox to allow Firefox to load extensions even when incognito: "split" is used in the manifest, with a fallback to "not_allowed". Extensions that do not rely on Chrome's "split" mode should remove the key from the manifest (or set the value to incognito: "spanning"`). This enables users to use the extension in private browsing mode, if they opt in to it.

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