Create _containsMarker to reduce redundant code

ext/bg/js/anki-note-builder.js

@@ -85,17 +85,7 @@ class AnkiNoteBuilder {
     }
 
     async injectAudio(definition, fields, sources, audioSystem, optionsContext) {
-        let usesAudio = false;
-        for (const fieldValue of Object.values(fields)) {
-            if (fieldValue.includes('{audio}')) {
-                usesAudio = true;
-                break;
-            }
-        }
-
-        if (!usesAudio) {
-            return true;
-        }
+        if (!this._containsMarker(fields, 'audio')) { return; }
 
         try {
             const expressions = definition.expressions;
@@ -114,17 +104,7 @@ class AnkiNoteBuilder {
     }
 
     async injectScreenshot(definition, fields, screenshot, anki) {
-        let usesScreenshot = false;
-        for (const fieldValue of Object.values(fields)) {
-            if (fieldValue.includes('{screenshot}')) {
-                usesScreenshot = true;
-                break;
-            }
-        }
-
-        if (!usesScreenshot) {
-            return;
-        }
+        if (!this._containsMarker(fields, 'screenshot')) { return; }
 
         const now = new Date(Date.now());
         const filename = `yomichan_browser_screenshot_${definition.reading}_${this._dateToString(now)}.${screenshot.format}`;
@@ -160,6 +140,16 @@ class AnkiNoteBuilder {
         return `${year}-${month}-${day}-${hours}-${minutes}-${seconds}`;
     }
 
+    _containsMarker(fields, marker) {
+        marker = `{${marker}}`;
+        for (const fieldValue of Object.values(fields)) {
+            if (fieldValue.includes(marker)) {
+                return true;
+            }
+        }
+        return false;
+    }
+
     static stringReplaceAsync(str, regex, replacer) {
         let match;
         let index = 0;