fix: convert zIndex type from String to Number

Author: hunterliu1003

dist/VueFinalModal.esm.js

@@ -12,7 +12,7 @@
       :overlay-transition="overlayTransition ? 'vfm' : ''"
       :z-index-auto="zIndexAuto"
       :z-index-base="zIndexBase"
-      :z-index="allowZIndex ? zIndexBase : false"
+      :z-index="allowZIndex ? zIndex : false"
       :attach="attach ? '#attach' : false"
       :focus-retain="focusRetain"
       :focus-trap="focusTrap"

dist/VueFinalModal.esm.js.map

@@ -127,7 +127,7 @@ export default {
     calculateZIndex() {
       if (this.zIndex === false) {
         if (this.zIndexAuto) {
-          return this.zIndexBase + 2 * (this.modalStackIndex || 0)
+          return +this.zIndexBase + 2 * (this.modalStackIndex || 0)
         } else {
           return false
         }