The --reporter value -> its class. Keyed by ReporterName, so adding a name to REPORTERS without wiring it up here is a type error rather than a silent fall back to tap.

Exported because the JS API instantiates the same four by name; a second copy of this table would mean a new reporter worked from the CLI and not from run({ reporter: … }).

new BUILT_IN_REPORTERS.tap(); // the same instance `--reporter=tap` builds