Everywhere: Replace single-char StringView op. arguments with chars

This prevents us from needing a sv suffix, and potentially reduces the
need to run generic code for a single character (as contains,
starts_with, ends_with etc. for a char will be just a length and
equality check).

No functional changes.

Userland/Libraries/LibWeb/CSS/Parser/Block.cpp

@@ -22,7 +22,7 @@ String Block::to_string() const
     StringBuilder builder;
 
     builder.append(m_token.bracket_string());
-    builder.join(" ", m_values);
+    builder.join(' ', m_values);
     builder.append(m_token.bracket_mirror_string());
 
     return builder.to_string();

Userland/Libraries/LibWeb/CSS/Parser/Function.cpp

@@ -23,9 +23,9 @@ String Function::to_string() const
     StringBuilder builder;
 
     serialize_an_identifier(builder, m_name);
-    builder.append("(");
-    builder.join(" ", m_values);
-    builder.append(")");
+    builder.append('(');
+    builder.join(' ', m_values);
+    builder.append(')');
 
     return builder.to_string();
 }

Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -2513,7 +2513,7 @@ Optional<StyleProperty> Parser::convert_to_style_property(Declaration const& dec
             property_id = PropertyID::Custom;
         } else if (has_ignored_vendor_prefix(property_name)) {
             return {};
-        } else if (!property_name.starts_with("-")) {
+        } else if (!property_name.starts_with('-')) {
             dbgln_if(CSS_PARSER_DEBUG, "Unrecognized CSS property '{}'", property_name);
             return {};
         }

Userland/Libraries/LibWeb/CSS/Parser/Rule.cpp

@@ -25,11 +25,11 @@ String Rule::to_string() const
     StringBuilder builder;
 
     if (is_at_rule()) {
-        builder.append("@");
+        builder.append('@');
         serialize_an_identifier(builder, m_at_rule_name);
     }
 
-    builder.join(" ", m_prelude);
+    builder.join(' ', m_prelude);
 
     if (m_block)
         builder.append(m_block->to_string());