Replace unmaintained markdown parser library with a new one

[GuySartorelli]

Replaces erusev/parsedown with league/commonmark

The following 6 unit tests were failing prior to this change:

1) Doctum\Tests\Console\CommandHelpTest::testParseHelp
Failed asserting that two strings are identical.
/home/gsartorelli/dump/temp/cloned/doctum/tests/Console/CommandHelpTest.php:35

2) Doctum\Tests\Console\CommandHelpTest::testRenderHelp
Failed asserting that two strings are identical.
/home/gsartorelli/dump/temp/cloned/doctum/tests/Console/CommandHelpTest.php:57

3) Doctum\Tests\Console\CommandHelpTest::testUpdateHelp
Failed asserting that two strings are identical.
/home/gsartorelli/dump/temp/cloned/doctum/tests/Console/CommandHelpTest.php:72

4) Doctum\Tests\Console\CommandTest::testParseEndBeforeStart
Failed asserting that two strings are identical.
/home/gsartorelli/dump/temp/cloned/doctum/tests/Console/CommandTest.php:29

5) Doctum\Tests\Console\CommandTest::testRenderEndBeforeStart
Failed asserting that two strings are identical.
/home/gsartorelli/dump/temp/cloned/doctum/tests/Console/CommandTest.php:49

6) Doctum\Tests\Console\CommandTest::testUpdateCommandRun
Failed asserting that two strings are identical.
/home/gsartorelli/dump/temp/cloned/doctum/tests/Console/CommandTest.php:81

The remaining failures all look benign - the new package deals with HTML and whitespace differently than the old one. Specifically it doesn't trim whitespace, and it seems to adhere to the HTML5 spec and massages malformed HTML into valid HTML.

I've left the failing tests as they are so that you can see for yourself what the differences are. If you're happy with the changes let me know and I'll fix up the tests that are failing as a result of this change (i.e not the above 6).
Note that there are some configuration options for league/commonmark, but none of them include "don't fix broken HTML" or "trim whitespace" so using this library we won't get identical results to the previous one without significant effort.

[williamdes]

Thank you so much, you can go ahead with the tests

I doubt we will have much complaints

[GuySartorelli]

Done. As noted in the PR description the 6 remaining unit failures fail without this PR so I haven't touched those.

[williamdes]

Thanks, I will update the other unhappy tests
the link transform was useful, I may write some code to save it

[williamdes]

I am waiting on parsedown/parsedown#7 before updating the phar.
This PR was reverted (thanks for the typo fix!) and I used the new fork of the library.
See: f7b7362

Will keep you updated, if this goes nowhere I will go back to your solution 🙏🏻

[williamdes]

Seems to work, new fork helped me.
All tests are good since 44b0a18

[williamdes]

Symfony v8 support added, -dev phar released. Testing time !