#236 Feature/add invalid attribute to file upload

[franckdervaux]

🔗 Linked issue

Resolves #236.

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• [ ] 🐞 Bug fix (a non-breaking change that fixes an issue)
• [ ] 👌 Enhancement (improving an existing functionality like performance)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds invalid and error-text properties to the six-file-upload component.
It allows it to display error messages when it is invalid, for example if the selected file is too large.
It also supports an error-text slot for displaying more complex error messages.

📝 Checklist

• [x] I have linked an issue or discussion.
• [x] It's submitted to the main branch
• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] I have updated the documentation accordingly.
• [x] All tests are passing
• [x] New/updated tests are included
• [x] I have updated the "upcoming" section inside docs/changelog.md explaining the changes I contributed

If adding a new feature, the PR's description includes:

• [x] A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[stefanrufer]

@franckdervaux - as soon as you update the changelog according to the comment we will merge the PR.

[franckdervaux]

@stefanrufer commit https://github.com/six-group/six-webcomponents/pull/250/commits/0a5ec9a3255d7a8fd9bd21c6b8069c0b716c8ebf has the proper change I believe?

[davidemarcoli]

@stefanrufer @franckdervaux I've fixed the merge conflicts. I think it's ready now! Someone else has to approve it..