Add thread cancellation

[martinmoene]

Code

• jthread, with stop abilities
• stop_token
• stop_source
• stop_callback, make_stop_callback()
• condition_variable_any

Configuration

• ...

Tests

• jthread with stop abilities
• nostopstate_t, nostopstate,
• stop_token
• stop_source
• stop_callback, make_stop_callback
• condition_variable_any

Distribution

• vcpkg

Other

• ...