♻️(react) merge duplicated scss classes related to accessibility

'offscreen' and 'c__offscreen' were duplicated, merge these two classes
in a dedicated utils file.

packages/react/src/components/Forms/LabelledBox/index.tsx

@@ -33,7 +33,7 @@ export const LabelledBox = ({
         <label
           className={classNames("labelled-box__label", {
             placeholder: labelAsPlaceholder,
-            offscreen: hideLabel,
+            c__offscreen: hideLabel,
           })}
           htmlFor={htmlFor}
           id={labelId}

packages/react/src/components/Forms/Select/mono.spec.tsx

@@ -1569,7 +1569,7 @@ describe("<Select/>", () => {
         name: "City",
       });
       const label = screen.getByText("City")!.parentElement!;
-      expect(Array.from(label.classList)).toContain("offscreen");
+      expect(Array.from(label.classList)).toContain("c__offscreen");
     });
 
     it("renders menu empty placeholder when there are no options to display", async () => {

packages/react/src/components/Forms/Select/multi.spec.tsx

@@ -726,7 +726,7 @@ describe("<Select multi={true} />", () => {
         name: "Cities",
       });
       const label = screen.getByText("Cities")!.parentElement!;
-      expect(Array.from(label.classList)).toContain("offscreen");
+      expect(Array.from(label.classList)).toContain("c__offscreen");
     });
 
     it("is possible to select again the last deleted item", async () => {