chore(toys): Stop background processes using gen on_interrupt directive

[aandreassa]

Convenience tool to stop all processes spawned via toys gen (which may live in the background)

Repro steps:

1. Change gapic-generator/templates/default/service/rest/test/method/_normal.erb to reference an incorrect value (e.g. s/field.name/field.wrong_name)
2. toys gen language_v1.
3. Once process hangs, send interrupt signal.