Add support for DevTools to open external links

[tomgilder]

See flutter/devtools#3251 - external links are currently failing when DevTools are embedded in VSCode due to the "allow-popups" sandbox restriction being enabled.

There might be a simpler solution to this that I'm unaware of.

I'm not very experienced with VSCode/TypeScript development, please check carefully 😁

Especially not sure if I got the dispose logic right.

Buddy PR on DevTools: flutter/devtools#3252

[DanTup]

I added a note on flutter/devtools#3252 (review). If we can't come up with something better and DevTools is happy to have that change, I think this looks reasonable. It would be good to avoid leaking too much VS Code stuff into DevTools if we can help it though.

[DanTup]

I think there's one change we should make in case we need to extend in future, but otherwise looks good to me - thanks!

[DanTup]

Thanks! I'm OOO this week, but will check the dispose is all good next week and merge this for the next version :-)

[DanTup]

This all looks good to me, and seems to work fine. It does trigger an error because of the change in DevTools to also call allow the original event to be handled, but I think I agree with Jacob that it's better to always call that. It's unlikely the sandbox rules will change, so it should never trigger to windows being opened, and the error is explainable.

Thanks!