Save diff commands

[marywinter3]

The saveoutput command allows saving the output of any GDB command, either explicitly or by reusing the last executed one. The diffoutput command compares the current output of a command to its previously saved version and shows a diff if differences are found. The tests cover correct argument handling, saved output integrity, and edge cases like missing commands or outputs.

[peace-maker]

Closes #171

[disconnect3d]

@marywinter3 this needs an update, see CI builds failing