Add support for page title styles within editor-styles.

[kjellr]

Theme authors can customize the page title style by enqueuing a stylesheet that includes something like this:

.editor-post-title__block .editor-post-title__input {
	font-family: serif;
}

However, if their theme is using our auto-injected CSS for editor-styles (introduced in #9008), that's not possible.

It's very likely that theme authors will want the page title font/size to match up with the rest of the text in the editor styles, so we should figure out how to make that possible without enqueueing a second editor stylesheet.

cc @youknowriad

[nickcernis]

This may be solved by #10067 if body is mapped to .edit-post-visual-editor (which contains the .editor-post-title__input element) instead of .editor-block-list__block (which does not).

You would be able to do something like this in editor styles:

body .editor-post-title__block .editor-post-title__input {
	font-family: serif;
}

And Gutenberg would map it to this, which will affect the page title:

.edit-post-visual-editor .editor-post-title__block .editor-post-title__input {
	font-family: serif;
}

[nickcernis]

Noting this now works for me in the editor stylesheet as of WP 5.0 Beta 3:

.editor-post-title__block .editor-post-title__input {
	font-family: serif;
}

This can likely be closed as a result.

[youknowriad]

Awesome thanks