Improve merging of chained items

[amendlik]

Issue #, if available: N/A

Description of changes:

• Add a space between merged items
• Whitespace returned by ResponseBots is no longer required
• Enable chaining of plaintext and SSML items while maintaining SSML content
• Enable chaining of plaintext and Markdown items while maintaining Markdown content
• Honor SSML returned from ResponseBots

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