Avoid overwriting DomainSets' UnionDomain plot recipe (#985)

DomainSets v0.8.3 added DomainSetsRecipesBaseExt, which defines a plot
recipe for UnionDomain. ApproxFun's identical recipe overwrote it, which
is not permitted during precompilation and broke the Aqua tests. Only
define ApproxFun's recipe when the extension is not available.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: dlfivefifty

src/Plot/Plot.jl

@@ -121,12 +121,15 @@ end
     [real(a),real(b)], [imag(a),imag(b)]
 end
 
-@recipe function f(dd::UnionDomain)
-    @series component(dd,1)
-    for k=2:ncomponents(dd)
-        @series begin
-            primary := false
-            component(dd,k)
+# DomainSets v0.8.3 and later provide this recipe in DomainSetsRecipesBaseExt
+if Base.get_extension(DomainSets, :DomainSetsRecipesBaseExt) === nothing
+    @recipe function f(dd::UnionDomain)
+        @series component(dd,1)
+        for k=2:ncomponents(dd)
+            @series begin
+                primary := false
+                component(dd,k)
+            end
         end
     end
 end