[fix] Shift-F5 being falsely inserted as 'S-F5'

[JimZhouZZY]

What

Resolved the issue where Shift-F5 was incorrectly interpreted as S-F5 by adding a simple check: comb != "S-F5" during fallback.

Related: #2217

Why

Though this may not be the most elegant solution, it effectively addresses the issue for now, I think.

How to test your changes

1. Build and run the project
2. Enter math mode by typing $
3. Press Shift+F5 or Shift+F5+I or Shift+F5+O or Shift+F5+U or Shift+F5+N (possibly with the Fn key)
4. Verify that the integral sign is inserted, rather than the string S-F5

[JackYansongLi]

这个好，我看了没问题。等明天沈达再review一下。

BTW，师弟加个微信，haggittli

[JackYansongLi]

same problem as #2233

[JackYansongLi]

@da-liii macOS arm64的CI挂了，貌似和我本地端一个问题，看看是不是xmake的配置有问题