Terminal always show uppercase letters when using caps lock to switch input source

[jasonxia23]

Details

• Browser and browser version: Found in VSCode 1.50.1, I think it's browser unrelated.
• OS version: 10.15.7
• xterm.js version: Found in VSCode 1.50.1, I think it's xterm.js version unrelated.

Steps to reproduce

1. Set system input source to shuangpin, before which add shuangpin as an input source.
   
2. Lock caps lock, with caps lock unlock, you will input Chinese character, which is an expected behavior.
3. Input something in the terminal, I can only input upper case letters. But when I input something in some other application, I can input lower case letters.

[gqbre]

same issue：#2457

[Tyriar]

duplicate of #2457