Auto merge of #121877 - estebank:fancy-svg, r=compiler-errors

On tests that specify `--color=always` emit SVG file with stderr output

Leverage `anstyle-svg`, as `cargo` does now, to emit `.svg` files instead of `.stderr` files for tests that explicitly enable color output. This will make reviewing changes to the graphical output of tests much more human friendly.

<img src="https://raw.githubusercontent.com/rust-lang/rust/b4bdb56f86e136ca63bf71dca3034200c6c25900/tests/ui/error-emitter/highlighting.svg">

Author: bors