Add a more descriptive message for the error generated by an empty shorthand attribute

[rster2002]

Changes

Added an error message when the parser finds an empty shorthand attribute (#6086) Right now it's an error, but as the issue suggests, it could also changed to generate a warning instead of an error. Let me know what you think!

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[dummdidumm]

Not sure if this must be part of this PR and instead a separate PR but this code where a attribute value is read could be enhanced, too, to explicitely handle the {} case.

I would add both as errors for now, and in another step I think it would make sense to introduce a "loose" AST parsing mode for language tools which would try to only warn in these cases and continue parsing.

[rster2002]

In the original issue there was already a discusion about whether or not this should be an error or just a warning. I did not know how strict parser should handle these situations so I assumed it should be strict, but as you said: later this could be changed to be more lenient.