Add config for Bazel CI

This will ensure gRPC-Web is compatible with the latest Bazel.
If we enable presubmit testing, we can remove Bazel from Kokoro
and speed-up CI.

.bazelci/presubmit.yml

@@ -0,0 +1,20 @@
+---
+# TODO(yannic): Enable buildifier and test on Windows and RBE (both unsupported by rules_closure).
+platforms:
+  macos:
+    build_targets:
+      - //...
+    test_targets:
+      - //...
+
+  ubuntu1604:
+    build_targets:
+      - //...
+    test_targets:
+      - //...
+
+  ubuntu1804:
+    build_targets:
+      - "//..."
+    test_targets:
+      - //...