style: Use write_char in place of write_str when serializing single-character literals

Generated by running

  find servo/components/style -name "*.rs" -exec perl -p -i -e "s/write_str\(\"(.)\"\)/write_char('\1')/g" {} \;

(and then added `use std::fmt::Write;` in a couple of places to fix build errors that arose).

Differential Revision: https://phabricator.services.mozilla.com/D168217

components/style/queries/feature_expression.rs

@@ -263,7 +263,7 @@ impl ToCss for QueryFeatureExpression {
     where
         W: fmt::Write,
     {
-        dest.write_str("(")?;
+        dest.write_char('(')?;
 
         match self.kind {
             QueryFeatureExpressionKind::Empty => self.write_name(dest)?,