Snowpack: Select next available port if port is already taken

[FredKSchott]

Original Discussion: https://www.pika.dev/npm/snowpack/discuss/264
/cc @bjarkihall, @FredKSchott

Currently, we throw a gross-looking error if the 8080 port is taken. Instead, we should:

• [x] PR 1: Return well-formatted output, saying that the port is taken, and that they can use --port to specify a different port.
• [ ] PR 2: Instead of exiting, ask the user if it should use the next available port instead. If yes, do it.

[stramel]

@FredKSchott This will also need to modify the HMR port as well

[FredKSchott]

Great call. There's a PR to default to using the same port for HMR, which I think would solve for this as well, since we're only talking about this feature for the default "no explicit port defined" user.

https://github.com/pikapkg/snowpack/pull/365

[stramel]

For reference, something like this will end up being the final result. https://github.com/facebook/create-react-app/blob/c87ab79559e98a5dae2cd0b02477c38ff6113e6a/packages/react-dev-utils/WebpackDevServerUtils.js#L448

[stramel]

@jihchi Are you planning on tackling the second PR? If not, I will try tackling it.

[jihchi]

@stramel please go ahead.