refactor: use east const convention (#5409)

This change refactors the codebase to use the "east const convention", and adds a clang-format rule to follow this convention.

src/test/jtx/impl/amount.cpp

@@ -54,7 +54,7 @@ PrettyAmount::operator AnyAmount() const
 
 template <typename T>
 static std::string
-to_places(const T d, std::uint8_t places)
+to_places(T const d, std::uint8_t places)
 {
     assert(places <= std::numeric_limits<T>::digits10);