Add language endpoint to developer APIs (#41)

* Added languages endpoint and its tests

pages/api/v1/languages/[[...slug]].ts

@@ -0,0 +1,47 @@
+import { NextApiHandler } from "next";
+import NextCors from "nextjs-cors";
+import { retrieveFromType } from "@utils/language";
+
+type Data = {
+    languages: {
+        code: string,
+        name: string
+    }[]
+} | {
+    error: string
+};
+
+const methods = ["GET"];
+
+const handler: NextApiHandler<Data> = async (req, res) => {
+    await NextCors(req, res, {
+        methods,
+        origin: "*",
+        preflightContinue: true
+    });
+
+    const {
+        query: { slug },
+        method
+    } = req;
+
+    if (slug && Array.isArray(slug) && slug.length > 1)
+        return res.status(404).json({ error: "Not Found" });
+
+    if (!method || !methods.includes(method)) {
+        res.setHeader("Allow", methods);
+        return res.status(405).json({ error: "Method Not Allowed" });
+    }
+
+    const type = slug?.[0];
+
+    if (type !== undefined && type !== "source" && type !== "target")
+        return res.status(400).json({ error: "Type should be 'source', 'target' or empty" });
+
+    const langEntries = retrieveFromType(type);
+    const languages = langEntries.map(([code, name]) => ({ code, name }));
+
+    res.status(200).json({ languages });
+}
+
+export default handler;