Improve the shortcut `F5`, `F6`, `F7`, and `F8`

[YiqiXu]

In the math mode, we can type characters in normal upright shapes by using the shortcut F5. However, it currently affects only the character immediately following the F5 keypress. To improve this functionality, the following enhancements should be considered:

• The F5 shortcut should affect multiple characters in a row, similar to the behavior of the \math-up macro.
• It should also be applicable when selecting characters with the mouse drag.

Similarly, these suggestions can be extended to the shortcuts F6, F7, and F8.