Support Spanner schema and Skylark files.

Adds support for the .sdl file extension which is used for Spanner's
Schema Definition Language. It uses -- for comments in the manner of
.sql files.

Adds support for the .bzl file extension which is used for Bazel's
Starlark language. It uses # for comments in the manner of Python.

testdata/initial/file.bzl

@@ -0,0 +1,5 @@
+# Say hello
+def hello():
+  print("Hello world!")
+
+hello()