pw_assert: Split facade impl into separate file

This is a prerequisite for directly testing the assert macro facade
logic, without needing to swap out backends.

Change-Id: I436468d56b383a87708efe03b0bbad2d8d624904
diff --git a/pw_assert/BUILD.gn b/pw_assert/BUILD.gn
index ab55707..83c3207 100644
--- a/pw_assert/BUILD.gn
+++ b/pw_assert/BUILD.gn
@@ -23,7 +23,10 @@
 pw_facade("pw_assert") {
   backend = dir_pw_assert_backend
   public_configs = [ ":default_config" ]
-  public = [ "public/pw_assert/assert.h" ]
+  public = [
+    "public/pw_assert/assert.h",
+    "public/pw_assert/internal/assert_impl.h",
+  ]
   public_deps = [ dir_pw_preprocessor ]
 }