Enable a default set of mistune2 plugins for feature parity with mistune0

[dairiki]

The features provided by the mistune2 plugins url, strikethrough, footnotes, and table provide features which were supported by mistune 0.8.4 out of the box.

For the sake of backwards-compatibility, since these features were all supported by older versions of Lektor (due to use of mistune 0.x) we should enable these plugins by default when running with mistune2.

This PR does that.

Issue(s) Resolved

When using mistune >= 2, markdown features which were previously supported under mistune 0.x are disabled by default.

Related Issues / Links

This builds upon the work to support mistune 2 in PR #992.

Description of Changes

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

[nixjdm]

This is great.

Also, the test is the only Lektor plugin I can recall that uses on_markdown_lexer_config. I think it's worth adding a mention of it to the docs https://www.getlektor.com/docs/api/plugins/events/markdown-lexer-config/

[dairiki]

Also, the test is the only Lektor plugin I can recall that uses on_markdown_lexer_config. I think it's worth adding a mention of it to the docs https://www.getlektor.com/docs/api/plugins/events/markdown-lexer-config/

The test could just as well have used on_markdown_config in this case.

I'm not quite sure myself what the exact purpose of on_markdown_lexer_config is. The only real distinction distinction between the two is that on_markdown_lexer_config has access to the renderer instance, while on_markdown_config does not (since it is called before the renderer is constructed).

[nixjdm]

The test could just as well have used on_markdown_config in this case.

Ah, I see. Yeah, then I can't recall ever seeing a use of on_markdown_lexer_config.

I suppose someone could modify the render after it's created to add methods to it after creation. Perhaps that might seem more natural than customizing the markdown config and improved renderer that we have. That seems less explicit though, and maybe should be discouraged, in favor of supplying an explicitly made renderer to the config. Also off topic for this PR I suppose.