add strikethrough (somewhat broken)

client/src/app/markleft/markleft.scss

@@ -19,3 +19,7 @@ p {
 .format-underline {
     text-decoration: underline solid #000000 3px;
 }
+
+.format-strikethrough {
+    text-decoration: line-through;
+}

client/src/app/markleft/markleft.ts

@@ -36,7 +36,13 @@ const styles: Record<string, formatDescriptor> = {
     },
     U: {
         className: "format-underline"
-    }
+    },
+    S: {
+        className: "format-strikethrough"
+    } // TODO: some css style elements cannot simply be overlapped. strikethrough and underline both use the
+    // "text decoration" tag so simply applying both will overwrite one of the styles. some extra logic will have to
+    // be put in place to add a combined "format-strikethrough-formatunderline" style in which the text decoration
+    // contains both.
 }; // i.e. %B{...} will apply "format-bold"
 
 // blocks registry: this maps format specifiers to html block types