Automatically build and commit mediawiki/vendor (the output of composer) when running CI for the master branch of mediawiki/core. (This needs to later be extend to all extensions deployed on beta that add dependencies via the composer merge plugin.)
The reason this is needed is because we need a way to deploy to beta when we use composer during ci (T90303) instead of the vendor repository. Also automating this minimizes the window when core is broken after a change to vendor or the dependencies (T88211). In a world where we intend to actively split out common functionality in components making the use and update of components less painful is a good idea. This task would make T74952 unnecessary.