suggestion: rename `fresh` imports to `@fresh/core`

[iuioiua]

For 3 reasons:

1. Deviating from the norm as a JSR package might suggest that there's a special reason for doing so. I know this is what I thought but it seems that there isn't.
2. Having the scope visible suggests that there are other packages within the @fresh scope, which there are.
3. Having fresh as an import specifier instead of @fresh/core doesn't reflect JSR's suggested import statement. This could cause confusion. I imagine a beginner user wondering why they don't match.

The only reason I can think of for doing this is that it's less for the dev to type. But IMO, that reason's insufficient compared to counterarguments I list above.