refactor: Add simple clang-tidy readability checks (#6556)

This change enables the following clang-tidy checks:
-  readability-avoid-nested-conditional-operator,
-  readability-avoid-return-with-void-value,
-  readability-braces-around-statements,
-  readability-const-return-type,
-  readability-container-contains,
-  readability-container-size-empty,
-  readability-else-after-return,
-  readability-make-member-function-const,
-  readability-redundant-casting,
-  readability-redundant-inline-specifier,
-  readability-redundant-member-init,
-  readability-redundant-string-init,
-  readability-reference-to-constructed-temporary,
-  readability-static-definition

src/test/jtx/impl/amount.cpp

@@ -62,9 +62,13 @@ operator<<(std::ostream& os, PrettyAmount const& amount)
         if (n < c)
         {
             if (amount.value().negative())
+            {
                 os << "-" << n << " drops";
+            }
             else
+            {
                 os << n << " drops";
+            }
             return os;
         }
         auto const d = double(n) / dropsPerXRP.drops();