Implement Snap Packaging for libxml2

This merge request implements the necessary packaging details to release libxml2 as a snap. It provides easy access to the latest release/development snapshots of libxml2 command-line utilities like xmllint and xmlcatalog for multiple GNU+Linux distributions and processor architectures.

An unofficial snap has been built on the Snap Store at https://snapcraft.io/libxml2-utils, and can be installed after installing snapd by running the following command:

# snap install libxml2-utils

After installation you may gain access to the following commands:

• xmllint
• xmlcatalog

If you're willing to take over the maintenance of the snap, merge this MR and create a developer account at the Snap Store to start the transfer process.

Thank you.

Refer-to: Snapcraft - Snaps are universal Linux packages https://snapcraft.io/
Refer-to: Lin-Buo-Ren/libxml2-snap: Unofficial Snap Packaging for XML Toolkit from the GNOME Project https://github.com/Lin-Buo-Ren/libxml2-snap
Signed-off-by: 林博仁(Buo-ren, Lin) Buo.Ren.Lin@gmail.com