syntax: lua: require at least one digit for hexadecimal numbers

Since a "0x" by itself is invalid.  Also add word boundaries, so that
e.g. "00x1" (which is invalid) does not get partial highlighting.

Also remove some redundant backslashes from the strings regex.
Signed-off-by: Tom Levy <tomlevy93@gmail.com>

syntax/lua.nanorc

@@ -57,8 +57,8 @@ color green        "\-\-.*$"
 color green         start="\-\-\[\[" end="\]\]"
 
 # Strings
-color red "\"(\\.|[^\\\"])*\"|'(\\.|[^\\'])*'"
+color red ""(\\.|[^"\])*"|'(\\.|[^'\])*'"
 ##color red         start="\[\[" end="\]\]"
 
 # Hex literals
-color red "0x[0-9a-fA-F]*"
+color red "\<0x[0-9a-fA-F]+\>"