Introduce new flag `--incompatible_string_join_requires_strings`

https://github.com/bazelbuild/bazel/issues/7802

RELNOTES:
  A new flag `--incompatible_string_join_requires_strings` is introduced. The sequence argument of `string.join` must contain only string elements.
PiperOrigin-RevId: 240775005
diff --git a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
index ebe9004..91ab07b 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/SkylarkSemanticsConsistencyTest.java
@@ -155,6 +155,7 @@
         "--incompatible_no_transitive_loads=" + rand.nextBoolean(),
         "--incompatible_remap_main_repo=" + rand.nextBoolean(),
         "--incompatible_remove_native_maven_jar=" + rand.nextBoolean(),
+        "--incompatible_string_join_requires_strings=" + rand.nextBoolean(),
         "--incompatible_use_toolchain_providers_in_java_common=" + rand.nextBoolean(),
         "--internal_skylark_flag_test_canary=" + rand.nextBoolean(),
         "--incompatible_do_not_split_linking_cmdline=" + rand.nextBoolean());
@@ -200,6 +201,7 @@
         .incompatibleNoTransitiveLoads(rand.nextBoolean())
         .incompatibleRemapMainRepo(rand.nextBoolean())
         .incompatibleRemoveNativeMavenJar(rand.nextBoolean())
+        .incompatibleStringJoinRequiresStrings(rand.nextBoolean())
         .incompatibleUseToolchainProvidersInJavaCommon(rand.nextBoolean())
         .internalSkylarkFlagTestCanary(rand.nextBoolean())
         .incompatibleDoNotSplitLinkingCmdline(rand.nextBoolean())