Encodings::fromLaTeXCommand: if the command directly maps an entry of unicodesymbols...

Encodings::fromLaTeXCommand: if the command directly maps an entry of unicodesymbols, use it and bypass most of the logic.

This is important for commands like !`, that are equivalent to \textexclamdown. However, ! is matched earlier, because the logic works with prefixes, hence the output doesn't make sense.