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/libxrpl/protocol/STNumber.cpp

@@ -198,9 +198,13 @@ partsFromString(std::string const& number)
     {
         // we have an exponent
         if (match[6].matched && (match[6] == "-"))
+        {
             exponent -= boost::lexical_cast<int>(std::string(match[7]));
+        }
         else
+        {
             exponent += boost::lexical_cast<int>(std::string(match[7]));
+        }
     }
 
     return {mantissa, exponent, negative};