Add "Search bookmarks" string to Strings.swift

Add a new string "Search bookmarks" to Strings.swift for the bookmarks panel. This is a new feature that was added in #32469, but we chose to go with the same string as the history search temporarily so we could release the feature without waiting for translations.

Once translations have been obtained, this string should be used in the UI.

Add this string under the Bookmarks struct in Strings.swift:

        public struct Search {
            public static let SearchPlaceholder = MZLocalizedString(
                key: "Bookmarks.Search.Placeholder.vxxx", // Use the current version in version.txt
                tableName: "Bookmarks",
                value: "Search bookmarks",
                comment: "Placeholder text for the search field in the bookmarks panel, used to filter bookmarks by title or URL.")
        }

ℹ️ Reference Person @ih-codes

The l10n team will need to approve this new string as well.

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