fix: XML comment parse logics for block element that is adjacent to markdown content

[filzrev]

This PR intended to fix #10965

I've added additional code to handle case that <code> element is adjacent to markdown content.

In this case,
It need to insert extra newline, and it need to copy indent chars for TrimEachLine.

[filzrev]

Commit (057387d) fix issue reported at #10965 (comment)

When HTML tag exists before <pre>.
It require empty line before <pre> block.

Without this empty line <pre> tag code block seems to be handled as markdown by Markdig parser.

Minumum code that reproduce problem with LinqPad
It need to insert empty line between </p> and <pre> tags.

var markdown = Markdig.Markdown.ToHtml(@"
<p>Paragraph</p>
<pre><code class=""lang-csharp"">public class Sample
{
    // line1

    // line2
}</code></pre>");

markdown.Dump();

[filzrev]

Commit (2c6d4d3) fix issue when parent tag exists before/after <code> tag.
And refactor XML comment processing code.

[filzrev]

Latest commit (3b19f12) fix issue when <pre> tag and markdown text exists on same line.