Comment PRs with updated schedule information (#1576)

This adds a GH action to add a comment to every PR giving the updated
course schedule with the PR merged.

To accomplish this, I broke `mdbook-course` into a library and two
binaries, allowing the mdbook content to be loaded dynamically outside
of an `mdbook build` invocation.

I think this is a net benefit, but possible improvements include:
* diffing the "before" and "after" schedules and only making the comment
when those are not the same (or replacing the comment with "no schedule
changes")
* including per-segment timing behind `<details>` (with a few minutes
effort I couldn't get this to play nicely with the markdown lists)

---------

Co-authored-by: Martin Geisler <mgeisler@google.com>

mdbook-course/src/lib.rs

@@ -0,0 +1,19 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+pub mod course;
+pub mod frontmatter;
+pub mod markdown;
+pub mod replacements;
+pub mod timing_info;