Add a Rust implementation of the Perl preprocessing logic

This implementation uses a full HTML parser (Servo's, html5ever) and operates on a DOM. It ends up being somewhat more verbose than Perl, but hopefully also more extensible/maintainable.

For now, this is hidden behind the PROCESS_WITH_RUST environment variable. That environment variable is used by default for the Docker path, but not otherwise. Subsequent work will remove the Perl implementations entirely.

Author: jeremyroman

.dockerignore

@@ -4,3 +4,6 @@
 !*.pl
 !build.sh
 !lint.sh
+!Cargo.lock
+!Cargo.toml
+!src

.gitignore

@@ -4,3 +4,9 @@ html/
 output/
 mdn/.id-list
 mdn/developer.mozilla.org/
+highlighter/
+
+
+# Added by cargo
+
+/target