The site's current timezone offset is an important piece of information for any REST API client that needs to manipulate dates. It has not been previously available.
Expose both the `gmt_offset` (the site's current offset from UTC in hours) and `timezone_string` (which also provides information about daylight savings time) via the "site info" endpoint (the base `/wp-json` response).
Also update the `wp-api-generated.js` fixture file with the changes to the default API responses.
Props sagarkbhatt.
Fixes#39854.
git-svn-id: https://develop.svn.wordpress.org/trunk@40238 602fd350-edb4-49c9-b593-d223f7449a82