serve/watch with --open blocks change watcher

[johannst]

In case the watch feature is enabled and we run one of the following commands

mdbook serve --open
mdbook watch --open

listening for file changes is blocked until the browser is closed.

Should we open the browser in a separate thread and start listening for changes immediately?

See PR (#1638) for example.

[johannst]

As discussed in PR (#1638) we can move to opener, which opens in background by default without waiting for the child process to finish.

I just took another look in open and saw that there is also that_in_background.

Using that_in_background we would need to take care of joining the thread handle.

I can create a new PR, any preference on staying with open or switching to opener?

[ehuss]

I would probably lean towards using opener. That is what we use in Cargo, so I am more familiar with it from there.

[johannst]

Created new PR (#1656), switching to opener.