popup: Implement TextPopupElement view for MAUI

[mstefarov]

TextElementPopupView is coming to MAUI!

• Text maps to Labels with formatted Spans.
• Blocks (e.g. paragraphs, table cells, list items) map to StackLayouts.
• Lists map to grids (one column for markers, another for content).
• Tables map to grids (with some extra logic to handle row/column assignment and rowspans/colspans).
• Images are loaded through RuntimeImage to take advantage of our authentication and caching stack.

Known limitations

• Text occasionally has extra space between lines or words. This is due to MAUI implementation being more sensitive to inconsistent newlines/whitespace than WPF's FlowDocument-based viewer. I am a preparing a follow-up PR that adds a document tree simplification pass to the HTML parser that will address this.
• Text links are not clickable on some platforms due to Gestures don't work on Label Spans dotnet/maui#4734 (might be fixed before NET 8 GA)
• Subscript (<sub>) and superscript (<sup>) do not have any visual effect, because MAUI does not expose any API to control this font feature.
• Table columns are always equally-spaced, which might inefficiently distribute space between columns with different amounts of content. "Auto" column width does not work well in MAUI because it prevents line-wrapping and causes the table to stretch out-of-bounds.

[dotMorten]

Looking great. Really only found one minor oversight

[dotMorten]

Note that his can also begin with data:image/. See

arcgis-maps-sdk-dotnet-toolkit/src/Toolkit/Toolkit/UI/Controls/PopupViewer/PopupMediaView.cs

Lines 89 to 92 in 1e8ad8a

	if (sourceUrl.StartsWith("data:image/"))
	{
	// might be base64
	var idx = sourceUrl.IndexOf(";base64,");

[mstefarov]

Looks like I missed this from both MAUI and WPF text element views. Fixed!

[mstefarov]

Tested on iOS and everything looks good except for cases where a popup contains nothing but text-only TextPopupElements. In cases like that, ScrollView does not correctly expand to show contents, and stays collapsed:

This is not a regression in this PR -- it's probably due to a MAUI bug dotnet/maui#15374