Active text-level formatting is lost when selecting a new block-level formatting option

When selecting a new block-level formatting option (such as heading or list), any active text-level formatting options on the cursor are lost.

##### Steps to Reproduce

1. Enable one or more text-level formatting options, such as bold or italics.
2. Select heading, quote, code block, or bulleted/numbered list.

Here's a video demonstrating the issue:

https://user-images.githubusercontent.com/11605395/120843336-d8b17300-c522-11eb-86ae-928b6a652b15.mov

**Expected result**

Seeing as major text-editors like Google Docs and Microsoft Word (also Slack) preserve formatting choices like bold and italics when making a new list, I would expect Trix to do so as well.

##### Details

* Trix version: 1.3.1
* Browser name and version: Chrome 91, Safari
* Operating system: macOS, Android, iOS

(This issue doesn't appear to be platform-specific)

Start by reproducing the formatting loss in Trix 1.3.1 using the listed block-level options and browsers, then trace the handling of active text formatting when a new block is selected. Done means bold or italic formatting remains active after choosing a heading, quote, code block, or bulleted or numbered list.

Written by the indexing model from the issue text.