Show invalid URL message when link validity is broken.

Prior to this the text field would turn red, but the "Not a valid URL"
message did not appear to the user.

src/exports/elements/tip-tap-editor.ts

@@ -351,6 +351,7 @@ export class TipTapEditor extends TipTapEditorBase {
     } catch (error) {
       inputElement.setCustomValidity("Not a valid URL");
       this.__invalidLink__ = true;
+      inputElement.reportValidity();
       return;
     }