Feature request: Export as .tar.xz

7zip is packaged differently on different Linux distros, and ends up available via different command names as a result (i.e. it's not always `7z`). `.zip` archives generally end up significantly larger, and exporting as a raw directory and then explicitly archiving it via another command is extra hassle.

It would therefore be nice to be able to export a `.tar.xz` tarball, and benefit from the wide availability of `tar` and the same compression effectiveness as `.7z`.

This looks to be a pretty simple change - the `cmake -E tar` command that's used to make `.zip` and `.7z` archives can make `.tar.xz`, too. Right now it's used like
```
cmake -E tar cf archive_path.7z --format=7zip -- path/to/directory
```
and
```
cmake -E tar cf archive_path.zip --format=zip -- path/to/directory
```
and a `.tar.xz` can be made with
```
cmake -E tar cfJ archive_path.tar.xz -- path/to/directory
```
optionally with `--format=gnutar` if that makes things easier.

Start by tracing the export path that invokes `cmake -E tar` for `.zip` and `.7z` archives; the issue does not name a source file or test. Add `.tar.xz` export support using the described tar invocation, then verify that an exported archive is created and can be extracted.

Written by the indexing model from the issue text.