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/Applications/Spreadsheet/Writers/XSV.h

@@ -140,7 +140,7 @@ private:
         auto string = String::formatted("{}", FormatIfSupported(entry));
 
         auto safe_to_write_normally = (m_behaviors & WriterBehavior::QuoteAll) == WriterBehavior::None
-            && !string.contains("\n")
+            && !string.contains('\n')
             && !string.contains(m_traits.separator);
 
         if (safe_to_write_normally) {