feat: run saber server on 3001 port if 3000 is unavailable

[nblthree]

Summary
Closes #182
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• Bugfix
• Feature
• Code style update
• Refactor
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• No

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The PR fulfills these requirements:

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[nblthree]

Nice

[kidonng]

In a rare case, I run saber and it automatically "switched" to a new port after reloading (I didn't run any other server at the same time). But actually it still serves the pages on port 3000. It doesn't bother a lot so I don't open an issue for this.

~\..\..\the-story: master ≣ +1 ~6 -1 | ~5 -0 ! > saber
info Using config file: saber-config.js
info Using theme: ./theme
info Using 20 plugins
success Compiled client successfully in 14s!
info Available at http://localhost:3000
info 165 MB memory used
warning Found a change in your Saber config file, restart server to see the effect.
info (21:3:37) Rebuilding client due to changes in ~\Documents\Projects\the-story\.saber\config.json
success Compiled client successfully in 282ms!
warning Port 3000 is in use, switched to a new port
info Available at http://localhost:3001
info 93.7 MB memory used
info (21:5:4) Rebuilding client due to changes in ~\Documents\Projects\the-story\.saber\config.json
success Compiled client successfully in 257ms!
warning Port 3000 is in use, switched to a new port
info Available at http://localhost:3001
info 98.8 MB memory used

[nblthree]

@kidonng nice catch. found the problem will make a PR to fix it