icon: Rework icons to make use of the IconNamed trait

[andreaskrath]

This makes use of the IconNamed trait and a blanked implementation to convert anything that implements this to an Icon.

This allows for easily defined custom versions of IconName, while minimally changing existing code (essentially only if you previously made use of the .path() method on the IconName enum; this now requires an import of the IconNamed trait).

Example

use gpui_component::IconNamed;

pub enum IconName {
    Encounters,
    Monsters,
    Spells,
}

impl IconNamed for IconName {
    fn path(self) -> gpui::SharedString {
        match self {
            IconName::Encounters => "icons/encounters.svg",
            IconName::Monsters => "icons/monsters.svg",
            IconName::Spells => "icons/spells.svg",
        }
        .into()
    }
}

// this allows for the following interactions (works with anything that has the `.icon(icon)` method
Button::new("my-button").icon(IconName::Spells);
Icon::new(IconName::Monsters); 

If you want to directly "render" a custom IconName you must implement the RenderOnce trait and derive IntoElement on the IconName.

use gpui::{IntoElement, RenderOnce};
use gpui_component::IconNamed;

#[derive(IntoElement)]
pub enum IconName {
    // The same as before
}

impl IconNamed for IconName {
    // The same as before
}

impl RenderOnce for IconName {
    fn render(self, _: &mut gpui::Window, _: &mut gpui::App) -> impl gpui::IntoElement {
        gpui_component::Icon::empty().path(self.path())
    }
}

// this allows for the following interaction
div()
    .child(IconName::Monsters)

Overall I think is an improvement to the existing way to do custom IconName implementations.

I am unsure if this change should also be reflected in the documentation on the section with "Icons & Assets", though I personally think it would make sense to highlight this way to do custom versions of IconName as it is considerably less involved than the current approach.

Closes #1627.

[huacnlee]

Thank you. I have also updated the docs for this.