Support `/dev/stdout` as a valid `--out` paramter

[mhegazy]

See Sébastien Pierre (@sebastien) #1226 (comment) for details:

tsc --out /dev/stdout example.ts
error TS5033: Could not write file '/dev/stdout': ESPIPE, invalid seek

[Gaelan]

👍

[RyanCavanaugh]

Unclear why we're calling seek -- accepting PRs that address that.