Add a "Write your story" filter.

[markjaquith]

fixes #5466

Description

Adds a write_your_story filter to match the enter_title_here filter, so the default block appender prompt can be customized.

How Has This Been Tested?

Verified that this updates the prompt:

add_filter( 'write_your_story', function() {
	return 'Say something';
});

Types of changes

1. New filter write_your_story.
2. <DefaultBlockAppender /> accepts placeholder argument.

[aduth]

👍

[gziolo]

Just noting that it is not going to work properly with the encoded characters. See related issue with the title prompt: #4933.