Add instumentation via mw.track to empty search experiment
Adds the following instrumentation to the empty search experiment.
- 'counter.MediaWiki.experiment.web.emptySearch.enabled' On every pageview with the extension loaded.
- 'counter.MediaWiki.experiment.web.emptySearch.impression' When the experimental search suggestions are first rendered.
- 'counter.MediaWiki.experiment.web.emptySearch.click' On search suggestion click.
Events increment a counter in statsv.
Bug: T372780