Fix indent on first list item (#482)

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This PR fixes

• Bug: NormalizeRenderer incorrectly renders - > p #482 (the issues I was originally looking at)
• NormalizeRenderer loses block quote formatting when used in lists #320 (the original report of the problem)

In short the Markdig.Renderers.Normalize.ListRenderer would include the text for the "list" part of the list item as text, and then push an Indent on the stack comprised on a string of spaces, and lastly then render all the children items.

This caused a problem when the first child block wanted to add its own Indent on the stack. When that first item was a QuoteBlock, previousWasLine within TextRendererBase would still be false, causing its own indent to be "eaten" on the first line of the QuoteBlock. As reported in the bug

- > p

Would get changed into

- p

At the same time if first child block was a sub list an extra new line would be inserted changing

- - a

Into

- 
  - a

To correct this problem, I used Markdig.Renderers.Roundtrip.ListRenderer as inspiration, and updated the Normalize version to move all of the "List" data into the Indent. This did require creating a new constructor so that we could create Indents that would have a specified "first line", and all subsequent uses of the Indent would yield a string the same width of the "first line", but made of spaces. Additional a new public method was add, PushHangingIndent to allow pushing these new Indents to the stack.

This mostly addressed the problem. However it created a problem for truly empty list items. They had no children, and as a result WriteChildren would effectively be a no-op. Again the Roundtrip version contains a solution, we detect this case and just write the empty string, which will respect the Indent.

There is one caveat with that, as noted in the two new tests ListUnorderedEmpty and ListOrderedEmpty. With this implementation, the trailing space that follow the OrderedDelimiter or BulletType is always included, even if it's not in the input. We could address this within our condition for the empty list, but I was not able to find a good way to detect the case when trailing space was in the input. I assume it's better to add the space that will likely be required instead of eating it. But that can easily be changed.

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This Null check looks like it might be a copy-paste error of
https://github.com/xoofx/markdig/pull/953/changes#diff-93312f2bb29ede6862c9f8cff9baf84ba584d58bd770d388a84381e3b523af22R183

And it should instead be checking to see if lineSpecific is null. But I'm not sure since this is very much looking out for the case when lineSpecific is null
https://github.com/xoofx/markdig/pull/953/changes#diff-93312f2bb29ede6862c9f8cff9baf84ba584d58bd770d388a84381e3b523af22R109

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Another difference I noticed we might want to drop this limited implication of ordered lists identifiers, and instead just use

markdig/src/Markdig/Renderers/Roundtrip/ListRenderer.cs

Line 31 in b07b98e

var bullet = listItem.SourceBullet.ToString();