Add language endpoint to developer APIs (#41)

* Added languages endpoint and its tests

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

@@ -0,0 +1,65 @@
+import httpMocks from "node-mocks-http";
+import handler from "@pages/api/v1/languages/[[...slug]]";
+
+it("returns 404 on >1 params", async () => {
+    const { req, res } = httpMocks.createMocks<any, any>({
+        method: "GET",
+        query: { slug: ["one", "two"] }
+    });
+
+    await handler(req, res);
+    expect(res.statusCode).toBe(404);
+});
+
+it("returns 405 on forbidden method", async () => {
+    const { req, res } = httpMocks.createMocks<any, any>({
+        method: "POST",
+        query: {}
+    });
+
+    await handler(req, res);
+    expect(res.statusCode).toBe(405);
+});
+
+it("returns 400 on wrong param", async () => {
+    const { req, res } = httpMocks.createMocks<any, any>({
+        method: "GET",
+        query: { slug: ["other"] }
+    });
+
+    await handler(req, res);
+    expect(res.statusCode).toBe(400);
+});
+
+it("returns 200 on empty param", async () => {
+    const { req, res } = httpMocks.createMocks<any, any>({
+        method: "GET",
+        query: {}
+    });
+
+    await handler(req, res);
+    expect(res.statusCode).toBe(200);
+    expect(res._getJSONData()).toStrictEqual({ languages: expect.any(Array) });
+});
+
+it("returns 200 on 'source' param", async () => {
+    const { req, res } = httpMocks.createMocks<any, any>({
+        method: "GET",
+        query: { slug: ["source"] }
+    });
+
+    await handler(req, res);
+    expect(res.statusCode).toBe(200);
+    expect(res._getJSONData()).toStrictEqual({ languages: expect.any(Array) });
+});
+
+it("returns 200 on 'target' param", async () => {
+    const { req, res } = httpMocks.createMocks<any, any>({
+        method: "GET",
+        query: { slug: ["target"] }
+    });
+
+    await handler(req, res);
+    expect(res.statusCode).toBe(200);
+    expect(res._getJSONData()).toStrictEqual({ languages: expect.any(Array) });
+});