Add token support to the Results views area plugin

Problem/Motivation

The 'Results summary' views area has a small set of tokens related to the view result, but doesn't allow the tokens that the text area provides.

Doing this along with ✨ Make exposed filter values available as tokens for text areas Needs work would allow things like a views header which says:

Your search for 'cake' returned 57 results.

Steps to reproduce

Proposed resolution

Change the Results class to inherit from TokenizeAreaPluginBase.

Note that 📌 Replace views results summary with text area Needs work proposes merging the tokens from the Results class into the Text class, which is a better long-term solution, but requires BC handling. This current issue might be a good stopgap.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet