chore: upgrade Mitt to 2.1 (#6287)

Fixes #6285.

vendor/README.md

@@ -7,7 +7,7 @@ Therefore we put all common dependencies into this directory, `vendor`. This mea
 The process for updating a vendored dependency is:
 
 1. `npm install {DEP NAME HERE}`
-2. `cp -r node_modules/DEP vendor/DEP`
+2. `cp -r node_modules/DEP vendor`
 3. Update `eslintrc.js` to forbid importing DEP directly (see the `Mitt` rule already defined in there).
 4. Use the new DEP, and run `npm run tsc` to check everything compiles successfully.
 5. If the dep ships as compiled JS, you may need to disable TypeScript checking the file. Add an entry to the `excludes` property of the TSConfig files in `vendor`. (again, see the entry that's already there for Mitt as an example). Don't forget to update both the ESM and CJS config files.

vendor/mitt/README.md

@@ -69,6 +69,9 @@ emitter.on('*', (type, e) => console.log(type, e) )
 // fire an event
 emitter.emit('foo', { a: 'b' })
 
+// clearing all events
+emitter.all.clear()
+
 // working with handler references:
 function onFoo() {}
 emitter.on('foo', onFoo)   // listen
@@ -99,6 +102,7 @@ const emitter: mitt.Emitter = mitt();
 #### Table of Contents
 
 -   [mitt](#mitt)
+-   [all](#all)
 -   [on](#on)
     -   [Parameters](#parameters)
 -   [off](#off)
@@ -112,6 +116,10 @@ Mitt: Tiny (~200b) functional event emitter / pubsub.
 
 Returns **Mitt** 
 
+### all
+
+A Map of event names to registered handler functions.
+
 ### on
 
 Register an event handler for the given type.