Avoid modifier release immediately followed by modifier press

[user202729]

Summary of changes

If the user wants to type an all-uppercase string e.g. ABC,

before this change, the following events will be sent

+shift
+A
-A
-shift
sync
(delay)
+shift
+B
-B
-shift
sync
(delay)
+shift
+C
-C
-shift
sync
(delay)

now we get the following, which is much more reasonable

+shift
+A
-A
sync
(delay)
+B
-B
sync
(delay)
+C
-C
-shift
sync
(delay)

I recall some issues (laggy Discord on Firefox?) that is suspected to be caused by this behavior (which is likely enough if some heavy logic are executed on shift key press/release), but I can't find the bug report anymore.

Caveat: not sure if news entry should go into linux or core, since I've only implemented it for linux.

Pull Request Checklist

• Changes have tests (this part isn't easy to unit test)
• News fragment added in news.d. See documentation for details

[mkrnr]

The change looks good to me, thanks!

Just a small one: better move the news file to "linux" instead of "core".

[mkrnr]

LGTM, thanks!

Feel free to merge when ready.