Update browser URL hash when user presses play button

[wch]

This closes #128. When the user presses the play button, it will automatically update the URL hash.

The encoding is done in a web worker, so it will not block the main thread.

A few remaining tasks:

• Don't auto-update the if files are very large -- need to figure out how to indicate to user in those cases that they should click the share button.
• Only update the URL if shinylive is in full-page mode on the examples/ or editor/ URL. Don't update if a shinylive app is embedded in another web page, or if someone is editing an app that they've created a static deployment for.
• Maybe don't auto-update if the code is loaded from a gist? Not sure what the behavior should be for this case.
• Only update the URL if the code has been changed (this might not be necessary, though).

[gadenbuie]

• need to figure out how to indicate to user in those cases that they should click the share button.

Maybe a toast message that says something like "Auto-updating the app link is disabled. Please use the share button."?

• Maybe don't auto-update if the code is loaded from a gist? Not sure what the behavior should be for this case.

Can gist and code coexist in the URL? I think the expectation is that copying the current URL will give you a link to the app you're editing right then. So to me it makes sense that if you open a gist, update the code and then copy the URL, that URL would restore the edited app.

Keeping the gist provenance seems reasonable so you could delete code=... to reset, but if they're mutually exclusive then I think it's okay to drop gist from the URL.

[wch]

• need to figure out how to indicate to user in those cases that they should click the share button.

Maybe a toast message that says something like "Auto-updating the app link is disabled. Please use the share button."?

That sounds reasonable -- I did think about that earlier and thought it would be too intrusive, but now that I think about it again, it would only happen in rare cases anyway.

• Maybe don't auto-update if the code is loaded from a gist? Not sure what the behavior should be for this case.

Can gist and code coexist in the URL? I think the expectation is that copying the current URL will give you a link to the app you're editing right then. So to me it makes sense that if you open a gist, update the code and then copy the URL, that URL would restore the edited app.

Keeping the gist provenance seems reasonable so you could delete code=... to reset, but if they're mutually exclusive then I think it's okay to drop gist from the URL.

gist and code can't coexist, so I suppose it does make sense to replace the gist= with code= if they re-run the app. Ideally it would only be replaced if the user actually modifies the files, and I'll look into making it work that way.