server: Refactor code block theme search into own service

src/services/code_block_theme.ts

@@ -0,0 +1,17 @@
+import fs from "fs";
+
+export function listSyntaxHighlightingThemes() {
+    const path = "node_modules/@highlightjs/cdn-assets/styles";
+    const allThemes = fs
+        .readdirSync(path)
+        .filter((el) => el.endsWith(".min.css"))
+        .map((name) => {
+            const nameWithoutExtension = name.replace(".min.css", "");            
+            
+            return {
+                val: `default:${nameWithoutExtension}`,
+                title: nameWithoutExtension.replace(/-/g, " ")
+            };
+        });
+    return allThemes;
+}