[PM-30266] fix(importer): improve Buttercup CSV import mapping

[majiayu000]

Summary

• Handle url field case-insensitively (URL, url, Url)
• Map note field to cipher notes
• Add !type to official props to exclude from custom fields
• Only add non-empty custom fields

Fixes #17119

Test plan

• Import a Buttercup CSV export with url (lowercase) field
• Verify URL is correctly imported to login URIs
• Import with note field
• Verify notes are correctly imported
• Verify !type, !group_id, id fields are not added as custom fields

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[bitwarden-bot]

Thank you for your contribution! We've added this to our internal tracking system for review.
ID: PM-30266
Link: https://bitwarden.atlassian.net/browse/PM-30266

Details on our contribution process can be found here: https://contributing.bitwarden.com/contributing/pull-requests/community-pr-process.

[harr1424]

Hi @majiayu000 👋 !

Thanks for your contribution, and especially for providing thorough test coverage. This link describes the process that your contribution will undergo before becoming a part of the Bitwarden codebase. Please keep in mind that your submission will be reviewed by Bitwarden's product and engineering teams before an official code review happens on Github. Since this review process occurs internally, it may seem as though little progress is taking place.

We understand many former Buttercup users have migrated to Bitwarden, and are very grateful for the improved support your PR will provide. Thank you for your patience throughout the review process, and please feel free to check in at anytime by tagging me here.