(core) Add our very own implementation of tokenfield (aka pillbox, tag list)

Summary:
- Supports auto-completion
- Supports various keyboard interactions to delete/select/copy-paste tokens
- Supports various mouse interactions to select and move tokens.
- Supports undo/redo while editing tokens.

Test Plan: stand-alone fixture and browser test verify the supported behaviors

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2789

test/nbrowser/gristUtils.ts

@@ -964,6 +964,10 @@ export async function copyKey() {
   return await isMac() ? Key.chord(Key.CONTROL, Key.INSERT) : Key.chord(Key.CONTROL, 'c');
 }
 
+export async function cutKey() {
+  return await isMac() ? Key.chord(Key.CONTROL, Key.DELETE) : Key.chord(Key.CONTROL, 'x');
+}
+
 export async function pasteKey() {
   return await isMac() ? Key.chord(Key.SHIFT, Key.INSERT) : Key.chord(Key.CONTROL, 'v');
 }