[JS] Use beforeinput event to trigger a keystroke event in the sandbox

- it aims to fix issue #14307;
 - this event has been added recently in Firefox and we can now use it;
 - fix few bugs in aform.js or in annotation_layer.js;
 - add some integration tests to test keystroke events (see `AFSpecial_Keystroke`);
 - make dispatchEvent in the quickjs sandbox async.

src/scripting_api/aform.js

@@ -466,6 +466,10 @@ class AForm {
 
     const event = globalThis.event;
     const value = this.AFMergeChange(event);
+    if (!value) {
+      return;
+    }
+
     const checkers = new Map([
       ["9", char => char >= "0" && char <= "9"],
       [
@@ -498,10 +502,6 @@ class AForm {
       return true;
     }
 
-    if (!value) {
-      return;
-    }
-
     const err = `${GlobalConstants.IDS_INVALID_VALUE} = "${cMask}"`;
 
     if (value.length > cMask.length) {
@@ -538,10 +538,6 @@ class AForm {
 
   AFSpecial_Keystroke(psf) {
     const event = globalThis.event;
-    if (!event.value) {
-      return;
-    }
-
     psf = this.AFMakeNumber(psf);
 
     let formatStr;