Fixes #1608, adds support for multiple alternate content sections

[kemsky]

#1608 Tested locally, seems to work.

AlternativeContent sections can also contain paragraphs, this is something that might need attention later.

[Copilot]

Pull Request Overview

This PR adds support for multiple AlternateContent sections within a Word processing run, addressing issue #1608. The change modifies the data structure to handle a collection of alternate content items rather than a single instance.

• Changes alternateContent field from single object to List collection
• Updates parsing logic to accumulate multiple AlternateContent sections
• Modifies serialization to iterate through all alternate content items

[Copilot]

The alternateContentField should be initialized to prevent null reference exceptions. Consider initializing it as 'new List<Vml.CT_AlternateContent>()' or ensure all usage checks for null.

[tonyqus]

Can you change the name to alternateContentFields?

[kemsky]

@tonyqus , done.

[Copilot]

The null check and list initialization happens inside the loop for each AlternateContent element. Consider initializing the list once outside the parsing loop or in the constructor to avoid repeated null checks.

Suggested change

	if (ctObj.alternateContent == null)
	{
	ctObj.alternateContent = new List<Vml.CT_AlternateContent>();
	}

[tonyqus]

LGTM