restore kover filters for BuildConfig and @Preview

The 0.7.x koverReport DSL was deleted during the 0.8.3 upgrade without
porting the filters, so coverage reports were silently including
BuildConfig classes and Preview composables. Restore them using the
0.8.x kover { reports { ... } } DSL.

Author: hopeman15

build.gradle.kts

@@ -56,3 +56,14 @@ dependencies {
     kover(project(":app"))
     kover(project(":slack-feedback"))
 }
+
+kover {
+    reports {
+        filters {
+            excludes {
+                classes("*BuildConfig")
+                annotatedBy("*Preview")
+            }
+        }
+    }
+}