Markdown text inconsistencies

There are numerous consistency issues between the JavaScript renderer and other renderers regarding Markdown-based content (like ignoring/preserving spaces, HTML entities, etc)

• Whitespace preservation
• HTML entities
• Newline preservation
• The fact that web visualizer supports things like headers and other markdown features

Repro payload

Repro payload

{
    "type": "AdaptiveCard",
    "body": [
        {
            "type": "TextBlock",
            "text": "Dogs    <\nCats\n\n# New paragraph"
        }
    ],
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.0"
}

As seen on web Visualizer...

As seen with UWP renderer...

Ignoring/preserving spaces

The JavaScript renderer doesn't preserve whitespace within TextBlock text. Other renderers, like UWP, do. This is a consistency issue, and also blocks proposals like #1078 working for monospace blocks (since those need whitespace preserved).

However, note that according to Markdown specs it seems like whitespace should be trimmed.

HTML entities

We should have consistent behavior here. Looking at CommonMark's specification of Markdown, HTML entities should be understood and processed by Markdown. Considering we are supporting markdown in our text blocks, we should be doing this?

Newline behavior

According to Markdown specs, a single new line should be trimmed and treated as a single whitespace character (thus not creating a new line). 2 or more new lines should be treated as the start of a new paragraph.