fix(icons/file): arcified folds

[karsa-mistmere]

Description

Added rounding to file-* fold corners & applied this change to other icons with folds.

Also merged, renamed & updated many file-* icons to have a more consistent feel, sticking to a guideline-compliant naming scheme.

• Moved most "external" addons to the bottom left corner and renamed them from -2 => -corner
• Removed most icon variants where the fold wasn't visible and added a backwards compatible alias for them

Before Submitting

• I've read the Contribution Guidelines.
• I've checked if there was an existing PR that solves the same issue.

[github-actions]

Added or changed icons

Preview cohesion