[SoftwareSerial] Enable rx-only or tx-only mode

I've been in several projects, where i've run out of available pins/interrups, but i've only needed RX or TX (not both), because i've used one-way communication. Can you please add some way to use only one pin like this?:

```cpp
#define DISABLED -1
SoftwareSerial myRXSerial(10, DISABLED); // RX, TX
SoftwareSerial myTXSerial(DISABLED, 11); // TX only does not need interrupts at all!
```

Start by locating the SoftwareSerial implementation and inspecting how its RX and TX pins are handled, including the DISABLED value shown in the examples. Confirm the expected behavior for RX-only and TX-only instances, then add coverage showing that each one-way construction works without requiring the unused pin or interrupt path.

Written by the indexing model from the issue text.