Shaders: add gcore-shaders and make graster-nodes no-std (#2925)

* gcore-shaders: add crate, move `color` and `blending` from gcore

* gcore-shaders: move `AsU32`

* gcore-shaders: move `ChoiceType`, switch `Cow` for `&str`, adjust node macro

* gcore-shaders: move `registry::types`

* gcore-shaders: move `context::Ctx`

* raster-nodes: make it `no_std` with `std` feature

* gcore-shaders: fix doctest

node-graph/gcore-shaders/src/context.rs

@@ -0,0 +1,9 @@
+pub trait Ctx: Clone + Send {}
+
+impl<T: Ctx> Ctx for Option<T> {}
+impl<T: Ctx + Sync> Ctx for &T {}
+impl Ctx for () {}
+
+pub trait ArcCtx: Send + Sync {}
+#[cfg(feature = "std")]
+impl<T: ArcCtx> Ctx for std::sync::Arc<T> {}