feature #3559 Allow CommonMark extensions to easily be added (kbond)

This PR was squashed before being merged into the 3.x branch.

Discussion
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Allow CommonMark extensions to easily be added

1. I discovered that commonmark 2 works just fine with `markdown-extra` so I added to the `require-dev` of `markdown-extra`'s `composer.json`
2. Added `LeagueMarkdownFactory` to `markdown-extra` as discussed in #3558
3. In `twig-extra-bundle`, if commonmark (and the above factory) is available, wire up the factory with extensions added via the `twig.markdown.league_extension` DI tag

If this PR would be acceptable, I can add some tests.

(closes #3558)

Commits
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4b86f5ef Allow CommonMark extensions to easily be added

Author: fabpot

composer.json

@@ -21,7 +21,7 @@
     "require-dev": {
         "symfony/phpunit-bridge": "^4.4.9|^5.0.9|^6.0",
         "erusev/parsedown": "^1.7",
-        "league/commonmark": "^1.0",
+        "league/commonmark": "^1.0|^2.0",
         "league/html-to-markdown": "^4.8|^5.0",
         "michelf/php-markdown": "^1.8"
     },