Move get element functionality to a helper (#33327)

Looking around on js components I found out many checks, different expressed but with same purpose.
Some of them are trying to parse string to element, others, jQuery element to js simple nodeElement etc

With this Pr, I am trying to give a standard way to parse an element

So this pr:

* Creates `getElement` helper that tries to parse an argument to element or null
* Changes `isElement` to make  explicit checks and return Boolean 
* fixes tests deficiencies

js/src/util/index.js

@@ -1,3 +1,5 @@
+import SelectorEngine from '../dom/selector-engine'
+
 /**
  * --------------------------------------------------------------------------
  * Bootstrap (v5.0.0): util/index.js
@@ -100,7 +102,29 @@ const triggerTransitionEnd = element => {
   element.dispatchEvent(new Event(TRANSITION_END))
 }
 
-const isElement = obj => (obj[0] || obj).nodeType
+const isElement = obj => {
+  if (!obj || typeof obj !== 'object') {
+    return false
+  }
+
+  if (typeof obj.jquery !== 'undefined') {
+    obj = obj[0]
+  }
+
+  return typeof obj.nodeType !== 'undefined'
+}
+
+const getElement = obj => {
+  if (isElement(obj)) { // it's a jQuery object or a node element
+    return obj.jquery ? obj[0] : obj
+  }
+
+  if (typeof obj === 'string' && obj.length > 0) {
+    return SelectorEngine.findOne(obj)
+  }
+
+  return null
+}
 
 const emulateTransitionEnd = (element, duration) => {
   let called = false
@@ -238,6 +262,7 @@ const execute = callback => {
 }
 
 export {
+  getElement,
   getUID,
   getSelectorFromElement,
   getElementFromSelector,