add codesign verify step for mac builds (#1509)

and use verbose output so that during release actions, someone can
get a little info to debug the issue even if they don't have a mac.

this ended up being implemented in the gradle build file instead of
the release.yml file, because the Build with Gradle step (which is
`./gradlew packageDistributionForCurrentOS`) includes both
signing and notarization. to stop the build early if the app signing
failed, the intervening check had to be inserted into the normal
build process.

app/build.gradle.kts

@@ -390,6 +390,25 @@ afterEvaluate{
     }
 }
 
+val verifySignedMacApp = tasks.register<Exec>("verifySignedMacApp") {
+    onlyIf { OperatingSystem.current().isMacOsX }
+    dependsOn("createDistributable")
+    group = "compose desktop"
+
+    commandLine(
+        "codesign",
+        "-vvv",
+        "--deep",
+        "--strict",
+        layout.buildDirectory.dir("compose/binaries/main/app/Processing.app").get().asFile.absolutePath
+    )
+}
+
+afterEvaluate {
+    tasks.named("notarizeDmg").configure {
+        dependsOn(verifySignedMacApp)
+    }
+}
 
 // LEGACY TASKS
 // Most of these are shims to be compatible with the old build system