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.sdl

@@ -0,0 +1,8 @@
+-- This is a fake table.
+CREATE TABLE Foo (
+  -- This is a fake column.
+  Bar INT64 NOT NULL,
+
+  -- Another fake column.
+  Baz STRING(MAX) NOT NULL,
+) PRIMARY KEY (Bar);