Fix bug #8306: LyX incorrectly imports mhchem commands outside math mode

Those commands can also be used in text mode but tex2lyx tries to
translate their argument causing havoc. So, import the whole commands
as ERT.

(cherry picked from commit ac92f2a79b)

lib/syntax.default

@@ -400,6 +400,8 @@ $$
 \bottomfraction
 \caption[]{}
 \cc{}
+\ce{}
+\cf{}
 \ccname
 \centering
 \centerline{translate}

status.20x

@@ -119,6 +119,8 @@ What's new
 - Fix support for mhchem upward arrows, which in mathed are rendered as
   empty (brace insets) superscripts (bug 8305).
 
+- Fix tex2lyx import of mhchem commands outside math mode (bug 8306).
+
 
 * USER INTERFACE