update search to use arrow function

[lao9]

Closes #329

Previous TypeError:

TypeError: Cannot read properties of undefined (reading 'request')
    at search (/path/to/node_modules/@notionhq/client/src/Client.ts:511:17)
    at iteratePaginatedAPI (/path/to/node_modules/@notionhq/client/src/helpers.ts:50:49)
    at iteratePaginatedAPI.next (<anonymous>)
    at example.example (/path/to/example.ts)

Passing in the listFn as client.search meant that this.request({...}) is unavailable unless search is an arrow function.

[wcauchois]

Great, thanks! Do you think this is worth a 2.1.1 release? 😄