feat: add google shopping offers (barcode → seller prices)

[0xGhostCasper]

What

Adds a new method client.google.shopping.offers({ barcode, gl?, hl? }) that calls GET /v1/google/shopping/offers.

It resolves a product by barcode (GTIN-8/UPC-A/EAN-13/GTIN-14) via Google web search, then returns that product's multi-seller Google Shopping offers:

const res = await client.google.shopping.offers({ barcode: "0190198001751", gl: "us" });
// { barcode, resolved_query, product_title, offers: [{ title, source, price, link, rating, ... }] }

• Costs 14 credits.
• 422 for an invalid/checksum-failing barcode, 404 when the barcode can't be resolved.

Changes

• src/google/shopping.ts — new offers() method, mirroring search/product/click.
• src/google/types.ts — new ShoppingOffersParams interface (barcode required, gl?, hl?).
• src/google/index.ts + src/index.ts — export ShoppingOffersParams.
• tests/google.test.ts — unit test asserting the endpoint path and forwarded params.
• docs/google.md, README.md, CHANGELOG.md — usage, method/credit tables, type list.
• Version bump 0.15.2 → 0.15.3.

Responses stay typed as GoogleResponse (Record<string, unknown>) to match every other Google method in the SDK — the repo does not define concrete response interfaces.

Test plan

• npm run typecheck — clean
• npm run build — success
• npm test — 211 passed (incl. new offers test)
• npm run lint — 7 pre-existing errors in src/youtube/types.ts (untouched by this PR); no new lint issues introduced