Make manual teaser break (<!--break-->) a configuration property

Problem/Motivation

The text.module allows - through its text_summary() function - content editors to manually define the portion of the text to be used as a summary.
The summary begins at the beginning of the text and
they can insert a marker (in the form of an HTML comment) at the place where the summary should end.
Traditionally, the marker used for that is <!---break-->.

That marker should be a configuration property for two reasons:

1. So that other parts of the system can request information about what the marker expected by text_summary() is; at the moment this information is hard-coded in a couple of places
2. So that the system can be configured to use other markers, in case <!--break--> needs to be avoided for whatever reason

Proposed resolution

Add a new configuration property to text.module that defines the summary marker.

Remaining tasks

Update text_summary() and text.settings.yml with the new configuration property.
Add an .install file to the text.module's code base with an update hook for adding the configuration property to existing systems.