Start adding integration tests

Add PHPUnit and an initial test.

Merge pull request #17 from wikisource/update-deps-2

Update PHP dependencies, and drop PHP 7

Merge pull request #14 from Parthiv-M/handle-mulwikisource

Handle mul wikisource pages

Merge pull request #13 from wikisource/no-key

Check for data path before using

Merge pull request #12 from wikisource/updates

Bump dependencies and add PHP 8 to CI

Merge pull request #11 from wikisource/wikidata-id-null

Avoid null value for Wikidata ID

Merge pull request #10 from wikisource/page-cache-key

Normalize cache key for Edition::fetchPageParse()

Merge pull request #6 from wikisource/indexpage-redirects

Follow redirects when finding subpages of Index pages

Merge pull request #5 from wikisource/upgrade

Upgrade dependencies

Merge pull request #2 from wikisource/better-pagelist

Add pagelist option that uses the API