Add monaco editor for json editing and diff view (#4)

Reviewed-on: #4

src/app/editor/editor.component.scss

@@ -2,10 +2,11 @@
   padding: var(--mat-sidenav-container-shape);
   display: flex;
   flex-direction: column;
+  box-sizing: border-box;
 }
 
-pre,
-.editor-items {
+.editor-items,
+.preview {
   flex: 1;
   overflow: auto;
 }
@@ -14,11 +15,16 @@ pre,
   padding: 16px;
 }
 
-.actions {
-  flex: 0 0 auto;
-}
-
 .proto-title {
   display: flex;
   justify-content: space-between;
 }
+
+mat-button-toggle-group {
+  width: fit-content;
+}
+
+ngx-monaco-editor,
+ngx-monaco-diff-editor {
+  height: 100%;
+}