This PR improves adding and working with icons by using the new
`DerivePathStr` to derive icon paths.
This means paths no longer need to be manually specified, and the
`IconName` and file name will always be consistent between icons.
This PR does not do any work to standardize icons visually, remove
unused icons, or any other such cleanup.
Release Notes:
- N/A
This is an update to 82 Zed UI and file_icons to improve icon legibility
and align contributed file_icons to the general icon system styling.
From review it appears they're 2px larger in width and height but gpui
render scaling is at play. The original icons were designed at 14x14 to
be viewed at that size, but the ui_font_size now controls this so
they've been updated to the default of 16x16 for optimum fidelity.

Release Notes:
- Updated various UI and file icons for consistency.