Keep inline formatting in list items and table cells

[trsdn]

Fixes #15.

Stacked on #28 — based on fix/17-nested-list-duplication, so review that one first. The diff shown here is only this change once #28 merges.

Problem

ConvertListItem and ConvertTable both read content via InnerText, which flattens all markup to plain text. The converter already handles <strong>, <em>, <code>, <a> and <s> properly — that pipeline was simply bypassed in the two places formatting appears most often.

Change

• List items and table cells now go through ConvertChildren, the same path used everywhere else.
• List items convert a copy of the <li> with nested lists detached, so nested content is still emitted separately as indented items rather than being inlined.
• Item and cell text is collapsed onto a single line, so a cell containing <br> or block children can no longer break the pipe table across rows.
• | inside a cell is escaped as \|, which previously split the cell into two columns.

Verification

Ran against the pinned HtmlAgilityPack 1.11.72.

Input	Before	After
<li>See <a href="…">docs</a> for <strong>details</strong></li>	- See docs for details	- See [docs](https://example.com) for **details**
<td><strong>Bold</strong></td>	| Bold |	| **Bold** |
<td>a|b</td>	| a|b | (breaks the table)	| a\\|b |

Nested-list output from #28 is unchanged, and existing task-list detection (☑/☐ prefix) still works because the prefix survives conversion.

dotnet build -c Release -r win-x64 succeeds with 0 warnings.

Scope

Text nodes are still emitted unescaped — tracked separately in #16.