[@vercel/edge] add header helpers (#8036)

* [@vercel/edge] add header helpers

* rename getIp => ipAddress, getGeo => geolocation, as suggested by @kikobeats

packages/edge/test/edge-headers.test.ts

@@ -0,0 +1,50 @@
+/**
+ * @jest-environment @edge-runtime/jest-environment
+ */
+
+import {
+  CITY_HEADER_NAME,
+  COUNTRY_HEADER_NAME,
+  Geo,
+  geolocation,
+  ipAddress,
+  IP_HEADER_NAME,
+  LATITUDE_HEADER_NAME,
+  LONGITUDE_HEADER_NAME,
+  REGION_HEADER_NAME,
+} from '../src';
+
+test('`ipAddress` returns the value from the header', () => {
+  const req = new Request('https://example.vercel.sh', {
+    headers: {
+      [IP_HEADER_NAME]: '127.0.0.1',
+    },
+  });
+  expect(ipAddress(req)).toBe('127.0.0.1');
+});
+
+describe('`geolocation`', () => {
+  test('returns an empty object if headers are not found', () => {
+    const req = new Request('https://example.vercel.sh');
+    expect(geolocation(req)).toEqual({});
+  });
+
+  test('reads values from headers', () => {
+    const req = new Request('https://example.vercel.sh', {
+      headers: {
+        [CITY_HEADER_NAME]: 'Tel Aviv',
+        [COUNTRY_HEADER_NAME]: 'Israel',
+        [LATITUDE_HEADER_NAME]: '32.109333',
+        [LONGITUDE_HEADER_NAME]: '34.855499',
+        [REGION_HEADER_NAME]: 'fra1',
+      },
+    });
+    expect(geolocation(req)).toEqual<Geo>({
+      city: 'Tel Aviv',
+      country: 'Israel',
+      latitude: '32.109333',
+      longitude: '34.855499',
+      region: 'fra1',
+    });
+  });
+});