Feathering for Gizmo lines

## What problem does this solve or what need does it fill?

The line gizmos (and everything built on top of lines) currently rely on external anti-aliasing to remove staircase effects. Since high-quality AA is relatively expensive, it would be nice if that wasn't necessary to get good-looking lines.

## What solution would you like?

Lines can implement "feathering", which creates extra vertices around the lines that fade out into transparency over a few pixels. egui/epaint is an example of a library that implements this.

## What alternative(s) have you considered?

- Use FXAA instead, which looks kinda bad and is still likely more expensive than feathering since it's a full-screen pass.
- Use higher-quality AA like MSAA, which is relatively expensive in terms of memory usage/traffic and costs extra resolve passes.

Start by locating Bevy's gizmo line-rendering entry points and tracing how line vertices are generated; compare the described approach with egui/epaint's feathering implementation. Done means line gizmos and features built on them visibly reduce staircase effects through transparent edge vertices without requiring external anti-aliasing.

Written by the indexing model from the issue text.