[lldb-dap] Minor change to stdio client name. (#141266)
Update the stdin <-> stdout client name to `stdio`. I noticed on macOS
if I get a crash report from lldb-dap the `/` in the client name causes
some thread names to be reported without the full name. For example
`Thread 6 Crashed:: */stdout.event_handler`. Switching to `stdio` I
think is still clear and not truncated in the macOS crash reports.
