LeagueMarkdown converter is explicitly requiring CommonMarkConverter

[trickeyone]

League CommonMark: v1.6.6
Twig Markdown-Extra: v3.3.3
Twig: v3.3.3

The LeagueMarkdown constructor is requiring the $converter parameter to explicitly be an instance of CommonMarkConverter, however with League CommonMark v1.6, the converter looks to have been split into two different instance types:

1. MarkdownConverter
2. CommonMarkConverter which extends MarkdownConverter

For our usage, we're needing to pass-in the $environment to the converter (GitHub flavor with some other slight modifications) So, with v1.6, we can't use CommonMarkConverter since it triggers a deprecation error:

Passing an $environment into the "League\CommonMark\CommonMarkConverter" constructor is deprecated in 1.6 and will not be supported in 2.0; use MarkdownConverter instead. See https://commonmark.thephpleague.com/2.0/upgrading/consumers/#commonmarkconverter-and-githubflavoredmarkdownconverter-constructors for more details.

Would it be possible to change the constructor for LeagueMarkdown to only require MarkdownConverter or maybe even just the ConverterInterface?

[stof]

Typehinting the MarkdownConverterInterface makes sense. Do you want to send a PR for that ?