Add properties that were previously omitted

PHP 8.3 (part of Ubuntu 24.04) outputs a deprecated warning for properties that
were not in the class definition. Error below. There were a few of these
missing from the `Ebook`, `GitCommit`, `CollectionMembership`, `EbookSource`,
and `Contributor` classes. Adding them doesn't change any functionality, but it
does make it clearer what properties a class has.

Some of these properties are never set for `Ebook` instances created from the
filesystem, i.e., `Created` and `Updated`, and some of them need to be manually
set to make `Ebook` instances from the filesystem and the database match, e.g.,
`GitCommitId`, `CollectionEbookId`, `EbookSourceId`, and `ContributorId`.
Making the `Ebook` instances from the filesystem and the database match each
other makes it easier to spot bugs in the future.

Previous errors with PHP 8.3:

```
PHP Deprecated:  Creation of dynamic property Ebook::$Created is deprecated in /standardebooks.org/web/lib/Traits/Accessor.php on line 42
PHP Deprecated:  Creation of dynamic property Ebook::$Updated is deprecated in /standardebooks.org/web/lib/Traits/Accessor.php on line 42
PHP Deprecated:  Creation of dynamic property GitCommit::$GitCommitId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$GitCommitId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$GitCommitId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$GitCommitId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$GitCommitId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property GitCommit::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property CollectionMembership::$CollectionEbookId is deprecated in /standardebooks.org/web/lib/Traits/Accessor.php on line 42
PHP Deprecated:  Creation of dynamic property CollectionMembership::$EbookId is deprecated in /standardebooks.org/web/lib/Traits/Accessor.php on line 42
PHP Deprecated:  Creation of dynamic property CollectionMembership::$CollectionId is deprecated in /standardebooks.org/web/lib/Traits/Accessor.php on line 42
PHP Deprecated:  Creation of dynamic property EbookSource::$EbookSourceId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property EbookSource::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property EbookSource::$EbookSourceId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property EbookSource::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property Contributor::$ContributorId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property Contributor::$EbookId is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
PHP Deprecated:  Creation of dynamic property Contributor::$SortOrder is deprecated in /standardebooks.org/web/lib/DbConnection.php on line 286
```

lib/GitCommit.php

@@ -2,6 +2,8 @@
 use Safe\DateTimeImmutable;
 
 class GitCommit{
+	public ?int $GitCommitId = null;
+	public ?int $EbookId = null;
 	public DateTimeImmutable $Created;
 	public string $Message;
 	public string $Hash;