Fix invalid URLs being returned from getRegistry, synchronize all copies of the function

[hippotastic]

Changes

• Fixes an issue reported in a Discord support thread.
• The getRegistry functions used in our CLI tools create astro, astro add and @astrojs/upgrade assumed that the output of the CLI command <package manager> config get registry would always be a URL. However, in the reported case, the command output started with a warning message, which was then returned by getRegistry without further validation, causing all dependent functionality to fail in unexpected ways.
• This PR validates the output by trying to construct a URL instance from it, which will either throw an error that gets caught by the catch clause, or have an empty host. In both cases, our existing fallback URL https://registry.npmjs.org will now be used.
• I also synchronized the slightly different 3 copies of the function as far as possible. They all use the caching logic that was already present in create-astro now, which could speed up the other CLI commands. In the future, we should investigate if we can actually use the same function from all tools instead of keeping 3 copies.

Testing

• I tested the changes locally and didn't encounter any issues.

Docs

• Just a bugfix, no docs should be needed.

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Latest commit: 77e297d

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[lilnasy]

Looks great to me, thanks for the quick PR!