change Diff to output <ins>, <del>

src/web/stylesheets/index.css

@@ -36,4 +36,5 @@
 @import "./layout/_structure.css";
 
 /* Operations */
+@import "./operations/diff.css";
 @import "./operations/json.css";

src/web/stylesheets/operations/diff.css

@@ -0,0 +1,8 @@
+del {
+    background-color: var(--hl3);
+}
+
+ins {
+    text-decoration: underline; /* shouldn't be needed, but Chromium doesn't copy to clipboard without it */
+    background-color: var(--hl5);
+}