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

[justadev-afk]

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.