Added syntax highlighting for XML Tools (#156)

Syntax highlight now should work on all tools apart from Mock Services.

Co-authored-by: Adam Bem <adam.bem@zoho.eu>
Reviewed-on: #156
Reviewed-by: Mikolaj Widla <widlam@noreply.example.com>

Frontend/assets/css/highlight.css

@@ -0,0 +1,69 @@
+.json-block {
+  height: 600px;
+  width: 97%;
+}
+
+.json-border {
+  border: 2px solid rgba(93, 99, 96, 0.705);
+  border-radius: 5px;
+}
+
+.json-border:focus {
+  box-shadow: 0 0 5px rgb(81, 203, 238);
+  border: 2px solid rgba(93, 99, 96, 0.705);
+  border-radius: 5px;
+}
+
+/*! Theme: Default Description: Original highlight.js style Author: (c) Ivan Sagalaev <maniac@softwaremaniacs.org> Maintainer: @highlightjs/core-team Website: https://highlightjs.org/ License: see project LICENSE Touched: 2021 */
+pre code.hljs{
+  display:block;
+  overflow-x:auto;
+  padding:1em
+}
+code.hljs{
+  padding:3px 5px
+}
+.hljs{
+  background:#FFFFFF;
+  color:#444
+}
+.hljs-comment{
+  color:#697070
+}
+.hljs-punctuation,.hljs-tag{
+  color:#444a
+}
+.hljs-tag .hljs-attr,.hljs-tag .hljs-name{
+  color:#444
+}
+.hljs-attribute,.hljs-doctag,.hljs-keyword,.hljs-meta .hljs-keyword,.hljs-name,.hljs-selector-tag{
+  font-weight:700
+}
+.hljs-deletion,.hljs-number,.hljs-quote,.hljs-selector-class,.hljs-selector-id,.hljs-string,.hljs-template-tag,.hljs-type{
+  color:#800
+}
+.hljs-section,.hljs-title{
+  color:#800;
+  font-weight:700
+}
+.hljs-link,.hljs-operator,.hljs-regexp,.hljs-selector-attr,.hljs-selector-pseudo,.hljs-symbol,.hljs-template-variable,.hljs-variable{
+  color:#ab5656
+}
+.hljs-literal{
+  color:#695
+}
+.hljs-addition,.hljs-built_in,.hljs-bullet,.hljs-code{
+  color:#397300
+}
+.hljs-meta{
+  color:#1f7199
+}
+.hljs-meta .hljs-string{
+  color:#38a
+}
+.hljs-emphasis{
+  font-style:italic
+}
+.hljs-strong{
+  font-weight:700
+}