Move dataset management from the corpora Vuex module to a Pinia store

Migrating the existing dataset state and actions to a Pinia store will require less work now than after #1294 (closed), #1295 (closed) or #1296 (closed), and will allow those issues to benefit from the API typing already done in !1487 (merged). This is also a step towards #1140 (closed), since we will need to clean up the corpora store module before we can migrate it to Pinia.