refactor: Update transaction folder structure (#6483)

This change reorganizes the `tx/transactors` directory for consistency and discoverability. There are no behavioral changes, this is a pure refactor. Underscores were chosen as the way to separate multi-words as this is the more popular option in C++ projects.
 
Specific changes:
- Rename all subdirectories to lowercase/snake_case (`AMM` → `amm`, `Check` → `check`, `NFT` → `nft`, `PermissionedDomain` → `permissioned_domain`, etc.)
- Merge `AMM/` and `Offer/` into `dex/`, including `PermissionedDEXHelpers`
- Rename `MPT/` → `token/`, absorbing `SetTrust` and `Clawback`
- Move top-level transactors into named groups: `account/`, `bridge/`, `credentials/`, `did/`, `escrow/`, `oracle/`, `payment/`, `payment_channel/`, `system/`
- Update all include paths across the codebase and `transactions.macro`

src/test/jtx/impl/AMM.cpp

@@ -5,8 +5,8 @@
 #include <xrpl/protocol/AmountConversions.h>
 #include <xrpl/protocol/ApiVersion.h>
 #include <xrpl/protocol/jss.h>
-#include <xrpl/tx/transactors/AMM/AMMHelpers.h>
-#include <xrpl/tx/transactors/AMM/AMMUtils.h>
+#include <xrpl/tx/transactors/dex/AMMHelpers.h>
+#include <xrpl/tx/transactors/dex/AMMUtils.h>
 
 namespace xrpl {
 namespace test {