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/ledger/BookDirs_test.cpp

@@ -56,8 +56,10 @@ struct BookDirs_test : public beast::unit_test::suite
         {
             auto AUD = gw["AUD"];
             for (auto i = 1, j = 3; i <= 3; ++i, --j)
+            {
                 for (auto k = 0; k < 80; ++k)
                     env(offer("alice", AUD(i), XRP(j)));
+            }
 
             auto d = BookDirs(*env.current(), Book(AUD.issue(), xrpIssue(), std::nullopt));
             BEAST_EXPECT(std::distance(d.begin(), d.end()) == 240);