add text zoom feature flag (#1057)

**Required**:

Task/Issue URL:
https://app.asana.com/0/inbox/392891325557408/1206268893832417/1208690165365832
iOS PR:  duckduckgo/iOS#3509
macOS PR:  duckduckgo/macos-browser#3496
What kind of version bump will this require?: Major

**Optional**:

Tech Design URL:
CC:

**Description**:
Adds feature flag for text zoom.  See platform PRs for testing.

Sources/BrowserServicesKit/PrivacyConfig/Features/PrivacyFeature.swift

@@ -61,6 +61,7 @@ public enum PrivacyFeature: String {
     case networkProtection
     case aiChat
     case contextualOnboarding
+    case textZoom
     case adAttributionReporting
 }