Replace degit with giget so that create package doesn't depend on Git being installed locally

[chrispcampbell]

There have been a couple more new users (on Windows) that have run into the issue described in #389 because they didn't already have Git installed on their machine. It looks like the create-astro package (which I used for inspiration for @sdeverywhere/create) started using the giget package as a replacement for degit. The giget package doesn't depend on Git being installed locally, and it is more likely to be maintained in the future than degit.

Instead of pursuing the error message and documentation changes in #389, I propose replacing degit with giget, which should offer the same functionality without the user having to worry about having Git installed locally.

[chrispcampbell]

Looks like giget works as advertised.

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Here's the success case:

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Here's what the error message looks like if the URL is invalid or the template can't be downloaded:

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Here's what the error message will look like if for some reason giget fails without reporting an error:

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I also updated the "init git repo" step so that it fails more gracefully if Git is not installed locally: