shell: shell_uart: reinstate multi-instance macro

Reinstate the `SHELL_UART_DEFINE` macro and moved the struct
declarations to header file, making it possible to create
another UART shell backend instance by doing
`SHELL_UART_DEFINE()` + `SHELL_DEFINE()` + `shell_init()`.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>