feat: drop axios/lodash/dotenv for native fetch, bump to 1.80.0 - #175

Remove the three runtime dependencies in favor of platform features:

- axios  -> native fetch (new request pipeline in src/base/request.base.ts
  with an HttpError that mirrors the previous axios error shape, so error
  handling and rate-limit retries are unchanged). Query params are serialized
  with URLSearchParams (repeated keys, skipping null/undefined). DataDragon
  and Seed now use fetch directly.
- lodash -> native JS (Object.entries-based champion id map, a small
  camelCase helper, object spreads instead of cloneDeep, direct property
  access instead of _.get/_.set).
- dotenv -> removed auto-loading; the live test uses a shared
  test/utils/loadEnv.ts helper instead.

Because the client now relies on a global fetch, the minimum supported
Node.js version is raised to >= 18 (engines field updated).

Also:
- Export the error classes from the package barrel (src/index.ts).
- Refactor README.md to be more visual and clearer.
- Enrich every example under example/ (comments, logging, error handling),
  fix stale imports/types and the removed Summoner.getByName usages.
- Bump version to 1.80.0 and regenerate yarn.lock.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>