add structure for simplifie creation of global command

app/fonctions/getPrefix.js

@@ -0,0 +1,16 @@
+const db = require("../db.js");
+
+module.exports = async function getPrefix(guildId) {
+  const prefix = await new Promise((resolve, reject) => {
+    db.get(
+      `SELECT prefix FROM prefix WHERE guildId = ?`,
+      [guildId],
+      (err, row) => {
+        if (err) reject(err);
+        resolve(row);
+      },
+    );
+  });
+
+  return prefix ? prefix.prefix : "!";
+};