[checks] Add checks for indexing of some operators

[frederick-vs-ja]

Due to some restrictions of LaTeX, these operators need to be escaped in indexing:

• operator& -> operator\&
• operator&& -> operator\&\&
• operator&= -> operator\&=
• operator% -> operator\%
• operator%= -> operator\%=
• operator~ -> operator\~{}
• operator^ -> operator\caret
• operator^= -> operator\caret=
• operator! -> operator"!
• operator!= -> operator"!=
• operator""suffix -> operator""""suffix
• operator| -> operator"|
• operator|| -> operator"|"|
• operator|= -> operator"|=

The current LaTeX source of working draft contains some unescaped forms in the indices, which makes some operators not correctly indexed in N5032.

Also, it's curious that unescaped | is correctly parsed in https://eel.is/c++draft/libraryindex.