Avoid port collisions

[himynameisdave]

Is your feature request related to a problem? Please describe.
Sapper currently attempts to run on PORT 3000 (or a provided PORT env var). Sometimes this means it will try to run on a PORT which is already in use, which causes headaches (see #920 && #730).

Describe the solution you'd like
It would be better if, after trying those ports first, it then used something like get-port to find an available port.

Describe alternatives you've considered
I think the Sapper creators have already tried mitigating this with an env variable, which is for sure a step in the right direction.

How important is this feature to you?
Low/Medium. It's a nice to have for folks who understand web development like myself, but I think this could be confusing for someone who is newer. It would be better if the system just found the port for you.

Additional context
Not sure what Sapper's policy is on adding new dependencies, but I would recommend that get-port one as it seems to be fairly well maintained, and I know I've used it in projects in the past with great success.

[swyxio]

fyi we already use port-authority

sapper/src/api/export.ts

Line 76 in 910d28e

const port = await ports.find(defaultPort);

which does the same job as get-port.

#920 is more about race condition.

#730 is a different issue yet again - not sure the cause.

but basically neither of these are solved by spawning yet more ports. we need to handle/wait for the ones we have better.

[himynameisdave]

@sw-yx You are right, thanks!