feat(RES-57): Add visibility to control scope of generated code (#18)

Add visibility parameter: public, package, internal, private
Impacted command: analytics, colors, fonts, images, strings, tags

Reviewed-on: #18

Sources/ResgenSwift/Strings/Stringium/Stringium.swift

@@ -72,7 +72,8 @@ struct Stringium: ParsableCommand {
                 inputFilename: options.inputFilenameWithoutExt,
                 extensionName: extensionName,
                 extensionFilePath: extensionFilePath,
-                extensionSuffix: options.extensionSuffix ?? ""
+                extensionSuffix: options.extensionSuffix ?? "",
+                visibility: options.extensionVisibility
             )
         }