Make the svg function to accept a path

[fogarecious]

Currently, to load an SVG file by the svg function, we have to call Handle::from_path.

use iced::widget::{svg, svg::Handle};
// ...
svg(Handle::from_path("path-to-pic.svg"));

A more ideal way is to pass the path directly into the svg function.

svg("path-to-pic.svg");

This PR enhances the convenience by adding From<T: Into<PathBuf>> to the Svg Handle. The modified code achieves consistency with the image counterpart.

[hecrj]

Thanks!