Properly handling letter modifiers

In LaTeX, writing \"a gives a small Latin letter a with diaeresis (umlaut). It is possible, but not necessary to wrap either the full expression or only the letter after the double quotation mark in curly brackets.

Beyond the double quotation mark, other modifiers are supported, such as single quotation marks, but also letters like c or u .

For example, \c{e} or \c e both give a small Latin letter e with cedilla, but what about \ce in correspondence to \"a ? Actually, \ce is interpreted as a command. The package mhchem provides one, for example.

This commit requires that modifiers, if being a letter, need to have either a space or some other separator like an opening curly bracket between it and the letter to be modified. If the input text is just \ce , this will be kept as verbatim LaTeX command.