Add `TransactionKernel::load_masm_source_files`

As mentioned in https://github.com/0xMiden/miden-base/pull/1789#discussion_r2299233877, we should expose `source_manager_ext::load_masm_source_files` as `TransactionKernel::load_masm_source_files`. This is useful in scenarios where users want to load kernel source files into a source manager but not assembler is present, such as when deserializing code.