Add a REST API to export site templates and template part as html files. When the REST API is requested, it responds by downloading a single ZIP file and exits early, without completing full request. To create the exported zip, the ZipArchive class is required. If this class is not present then the export will gracefully fail, returning a `WP_Error` object and 500 status error code.
Props spacedmonkey, youknowriad, Mamaduka, walbo, peterwilsoncc.
Fixes#54448 .
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