Throw if the user requests a port and it's already taken

[marvinhagemeister]

Before this PR we'd silently choose a different port, even if the user explicitely requested one via --port or process.env.PORT

[changeset-bot]

🦋 Changeset detected

Latest commit: 2f36c6f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
wmr	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Size Change: +87 B (0%)

Total Size: 692 kB

Filename	Size	Change
packages/wmr/wmr.cjs	660 kB	+87 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
examples/demo/dist/about/index.html	657 B	0 B
examples/demo/dist/assets/Calendar.********.css	702 B	0 B
examples/demo/dist/assets/style.********.css	386 B	0 B
examples/demo/dist/chunks/class-fields.********.js	201 B	0 B
examples/demo/dist/chunks/compat.********.js	15.3 kB	0 B
examples/demo/dist/chunks/index.********.js	200 B	0 B
examples/demo/dist/chunks/prerender.********.js	2.44 kB	0 B
examples/demo/dist/class-fields/index.html	635 B	0 B
examples/demo/dist/compat/index.html	1.49 kB	0 B
examples/demo/dist/env/index.html	712 B	0 B
examples/demo/dist/error/index.html	646 B	0 B
examples/demo/dist/files/index.html	675 B	0 B
examples/demo/dist/index.********.js	7.19 kB	0 B
examples/demo/dist/index.html	703 B	0 B
examples/demo/dist/lazy-and-late/index.html	658 B	0 B

_{compressed-size-action}

[marvinhagemeister]

Not sure what lighthouse is stumbling upon. Seems unrelated to this PR.