Support pagination via async iterator

[ilyazub]

Python wrapper for SerpApi has search.pagination(): serpapi/google-search-results-python#19.

pages = search.pagination(0, 20)

urls = []

for result in pages:
	urls.append(result['serpapi_pagination']['next'])

In JavaScript, such a paginator can be done using the async generator function.

const paginator = search.pagination({ start: 0, end: 20 });

const organicResults = [];

for await (const page of paginator) {
  // page contains a single paginated output.
  organicResults.push(...page.organic_results);
}

Reference implementation from aws-sdk-js-v3.

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Related to #6.

/cc @hartator @jvmvik