Add environment variables: SwiftBar launch time, last sleep time, last wake time

[melonamin]

As discussed in #276

[melonamin]

Variable	Value
SWIFTBAR_LAUNCH_TIME	SwiftBar launch date and time, ISO8601
OS_LAST_SLEEP_TIME	Last OS sleep date and time, ISO8601. Empty if OS didn't sleep since SwiftBar launch.
OS_LAST_WAKE_TIME	Last OS wake from sleep date and time, ISO8601. Empty if OS didn't sleep since SwiftBar launch.