Upgrade/Install: Detect the presence of the native PHP JSON extension before updating.

The PHP native JSON extension has been bundled and compiled with PHP by default since version 5.2.0. Because the minimum version of PHP required by WordPress is now 5.6.20 (see #46594 and [45058]), the related polyfills and workarounds have been removed (see [46205-46206,46208]).

However, even though the JSON extension is now included in PHP by default, it is still possible to disable the extension in a custom configuration. This change will prevent sites from upgrading if the JSON extension is disabled to prevent compatibility issues.

Props jrf, jorbin, dd32, desrosj.
Fixes #47699.
Built from https://develop.svn.wordpress.org/trunk@46455


git-svn-id: https://core.svn.wordpress.org/trunk@46253 1a063a9b-81f0-0310-95a4-ce76da25c4cd