Add 4 new browser snippets

triggerEvent - trigger an event on an element, insertAfter & insertBefore - parse and insert HTML after or before an element respectively, elementContains - checks if an element contains another element

snippets/elementContains.md

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+### elementContains
+
+Returns `true` if the `parent` element contains the `child` element, `false` otherwise.
+
+Check that `parent` is not the same element as `child`, use `parent.contains(child)` to check if the `parent` element contains the `child` element.
+
+```js
+const elementContains = (parent, child) => parent !== child && parent.contains(child);
+```
+
+```js
+elementContains(document.querySelector('head'), document.querySelector('title')); // true
+elementContains(document.querySelector('body'), document.querySelector('body')); // false
+```