Add ScopeId::ROOT (#1398)

* add ScopeId::ROOT

* replace ScopeId(0) with ScopeId::ROOT

---------

Co-authored-by: Jani Mustonen <jani.mustonen@taitounited.fi>

packages/core/src/scopes.rs

@@ -63,6 +63,21 @@ impl<'a, T> std::ops::Deref for Scoped<'a, T> {
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
 pub struct ScopeId(pub usize);
 
+impl ScopeId {
+    /// The root ScopeId.
+    ///
+    /// This scope will last for the entire duration of your app, making it convenient for long-lived state
+    /// that is created dynamically somewhere down the component tree.
+    ///
+    /// # Example
+    ///
+    /// ```rust, ignore
+    /// use dioxus_signals::*;
+    /// let my_persistent_state = Signal::new_in_scope(ScopeId::ROOT, String::new());
+    /// ```
+    pub const ROOT: ScopeId = ScopeId(0);
+}
+
 /// A component's state separate from its props.
 ///
 /// This struct exists to provide a common interface for all scopes without relying on generics.