Add support for skipping translation groups

This adds support for adding a comment of the form: `<!--
mdbook-xgettext: skip -->`. This will cause the system to skip the
next message group that would otherwise be translated.

It adds a dependency to the regex crate to match for the comment skip
pattern.

Tests were added, including tests that cover some odd situations
involving inline HTML, which unfortunately appear to be due to
pulldown-cmark/pulldown-cmark#712.

Author: dyoo

Cargo.lock

@@ -15,6 +15,7 @@ mdbook = { version = "0.4.25", default-features = false }
 polib = "0.2.0"
 pulldown-cmark = { version = "0.9.2", default-features = false }
 pulldown-cmark-to-cmark = "10.0.4"
+regex = "1.9.4"
 semver = "1.0.16"
 serde_json = "1.0.91"
 

Cargo.toml

@@ -182,6 +182,34 @@ Please see the [`publish.yml`] workflow in the Comprehensive Rust 🦀 repositor
 
 [`publish.yml`]: https://github.com/google/comprehensive-rust/blob/main/.github/workflows/publish.yml
 
+## Marking Sections to be Skipped for Translation
+
+A block can be marked to be skipped for translation by prepending a special HTML
+comment `<!--- mdbook-xgettext:skip -->` to it.
+
+For example:
+
+````markdown
+The following code block should not be translated.
+
+<!--- mdbook-xgettext:skip -->
+
+```
+fn hello() {
+  println!("Hello world!");
+}
+```
+
+Itemized list:
+
+- A should be translated.
+
+<!--- mdbook-xgettext:skip -->
+
+- B should be skipped.
+- C should be translated.
+````
+
 ## Normalizing Existing PO Files
 
 When mdbook-i18n-helpers change, the generated PO files change as well. This can