highlight: do not add html classes to unresolved local variables

highlight: do not add html classes to unresolved local variables

Eg. this avoid highlighting to only highlight the fist occurrence of `x`
in the following AST if no semantic analysis is run

~~~
var x = 5
print x
~~~

Now, either none, or both `x` are identified as variables according to how
mush semantic analysis is performed.

Signed-off-by: Jean Privat <jean@pryen.org>