LibWeb: Implement the "removeFormat" editing command

Author: https://github.com/gmta
Commit: aee8a75c40
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3216

Tests/LibWeb/Text/input/Editing/execCommand-removeFormat.html

@@ -0,0 +1,17 @@
+<script src="../include.js"></script>
+<div contenteditable="true">f<b>o</b>ob<i>a</i>r</div>
+<script>
+    test(() => {
+        const range = document.createRange();
+        getSelection().addRange(range);
+
+        const divElm = document.querySelector('div');
+        println(`Div contents: "${divElm.innerHTML}"`);
+
+        // Remove all formatting from 'foobar'
+        range.setStart(divElm, 0);
+        range.setEnd(divElm, 5);
+        document.execCommand('removeFormat');
+        println(`Div contents: "${divElm.innerHTML}"`);
+    });
+</script>