Fix TypeScript errors when using the truncateMixin

src/mixins.js

+import { defineComponent } from 'vue'
 import { TRUNCATE_LENGTHS } from './config'
 import { ensureArray } from './helpers'
 
@@ -6,7 +7,7 @@ const truncateFilter = maxLength => text => {
   return text ?? ''
 }
 
-export const truncateMixin = {
+export const truncateMixin = defineComponent({
   methods: {
     truncateSelect: truncateFilter(TRUNCATE_LENGTHS.select),
     truncateLong: truncateFilter(TRUNCATE_LENGTHS.long),
@@ -14,7 +15,7 @@ export const truncateMixin = {
     truncateNotification: truncateFilter(TRUNCATE_LENGTHS.notification),
     truncateId: truncateFilter(TRUNCATE_LENGTHS.id)
   }
-}
+})
 
 const corporaFilters = {
   // From a corpus, returns true if the corpus is readable.