Merge text node and segments

[JiuqingSong]

This change fixes two issues.

1. When two or more continuous text segments have the same format, we can merge them into one when normalizeContentModel and normalizeParagraph
2. When selection marker is located between two text segments that have same format, when create DOM tree, we can just create a single text node for them and remember the selection offset in context, and fixup their cached segments info.

This can solve the following problems:

1. Make the result content model predictable after changing selections
2. Correct the cached segments after Node.normalize() is called (https://outlookweb.visualstudio.com/Outlook%20Web/_queries/edit/309633/?triage=true)