Add stage metadata get/sets, prefix decl api.

* Add some stage metadata getters and setters.
* Prefix the experiemental declarative api with
  uppercase letters, to avoid clashes with the
  standard usd api:
  UsdStage → USDStage
  UsdPrim → USDPrim

DeclarativePixarUSD.usda

@@ -1,6 +1,6 @@
 #usda 1.0
 (
-    doc = "SwiftUSD v23.11.35 | Declarative API"
+    doc = "SwiftUSD v23.11.36 | Declarative API"
 )
 
 def "DeclarativeScene"

HelloPixarUSD.usda

@@ -1,6 +1,6 @@
 #usda 1.0
 (
-    doc = "SwiftUSD v23.11.35"
+    doc = "SwiftUSD v23.11.36"
 )
 
 def Xform "Geometry"

README.md

@@ -198,11 +198,11 @@ enum Creator
 
     /* Create a new USD stage with a transform and a sphere. */
 
-    UsdStage("HelloPixarUSD", ext: .usda)
+    USDStage("HelloPixarUSD", ext: .usda)
     {
-      UsdPrim("Hello", type: .xform)
+      USDPrim("Hello", type: .xform)
       {
-        UsdPrim("World", type: .sphere)
+        USDPrim("World", type: .sphere)
       }
     }
     .set(doc: "Stay Swifty.")

Sources/PixarUSD/Usd/Usd/Prim.swift

@@ -33,6 +33,7 @@ import Foundation
 import Usd
 
 public typealias UsdStageWeakPtr = Pixar.UsdStageWeakPtr
+public typealias UsdPrim = Pixar.UsdPrim
 
 public extension Usd
 {
@@ -95,7 +96,7 @@ public extension Usd
    * Please refer to ``Notice`` for a listing of the events that could cause
    * ``Notice/objectsChanged`` to be emitted.
    */
-  typealias Prim = Pixar.UsdPrim
+  typealias Prim = UsdPrim
   typealias StageWeakPtr = UsdStageWeakPtr
 }