Keep leading spaces in paragraphs when there's a list

[AfzalivE]

When there are no blockParserFactories, leading spaces should be preserved. Here's a use case:

Input

- one
  - two

When this is parsed with a Parser without any enabledBlockTypes, before this PR, it results in this:

- one
- two

But it should preserve the list hierarchy with the leading whitespace.

[AfzalivE]

@robinst Would you please be able to review this?

[AfzalivE]

Ping @robinst, would you please be able to review this? Thanks!

[robinst]

Note that this doesn't really have anything to do with lists, it's the same for a paragraph like this:

aaa
  bbb
    ccc

The reason for this is in the spec, leading spaces are skipped: https://spec.commonmark.org/0.31.2/#example-222
Final spaces in a paragraph are also skipped, see here: https://spec.commonmark.org/0.31.2/#example-226

Having said that, we should probably have a way to preserve these skipped spaces in the nodes somehow. Some ideas:

• Add a SkippedSpaces node that HTML rendering ignores but other renderers can make use of (e.g. the new MarkdownRenderer might also want to render it).
• Add some properties to Text nodes to indicate any leading/trailing spaces. Not sure how feasible this is with how inline parsing works. Not sure if Text would be enough, what if the line starts/ends with an inline element (e.g. emphasis, link etc).
• Add some properties to Paragraph nodes to indicate any leading/trailing spaces. This would have to store the line numbers somehow, and might not be super convenient to consume.

We also need to consider interaction with e.g. getMarkerIndent on ListItem. Maybe that could be replaced by whatever we build for this if it's generic enough.

[robinst]

Closing this as it's not the right approach, see comment above and #301.