Merge into Twig Template Suggester

As a maintainer of twigsuggest i would welcome the merger. Synergy and all that. Corresponding issue: #3057385: Merge themable_forms into twigsuggest

This sounds like a good idea. Seems silly to have special modules handle different template suggestions. I get the impression that Themable Forms is not being maintained, and it would be nice to get all form and field related template suggestions from one single module, because right now neither form.html.twig nor input.html.twig receives template suggestions from Themable Forms!

Ultimately I had to settle on using formdazzle! module for one project recently because this module lacked sensible template suggestions for form.html.twig (which is in the name of the module!). I was unaware that Twigsuggest existed and had support for form templates.

Suggested approach for merging functionality from an apparently unmaintained module into one you already maintain:

1. A maintainer of Twig Template Suggester should offer to take maintainership of Themable Forms module using the standard procedure.
2. Make it clear upfront that your intention is to merge the module functionality into the contrib module you already maintain, support both modules for some time, eg resolve some of the open issues in the queue, and then eventually mark the module as deprecated/replaced by the other module. Reference this issue to show that there is community support behind the merge.
3. You can also offer to add one or more maintainers of the module you wish to deprecate as co-maintainers of your module.

Then, assuming that goes through and you become a co-maintainer:

• Port the functionality from the module you wish to merge into the module you maintain, and cut a new release of the later, on the release notes, document the migration steps thoroughly, even if it is simply, require and enable the new module, disable and uninstall the old module.
• For community good will, consider reviewing and merging any of the open issues on the issue queue that contain patches, move them to your own issue queue, and fix them there.
• Update the project page of the module you're merging to mark it as "Replaced by Twig Template Suggester" (See Entityblock module project page for an example).
• Update the project page of your module to mention that functionality from the deprecated module has been merged, and give a shout-out to the previous maintainers.
• Optionally, cut a new release of the deprecated module to add a message to Drupal's Admin Report page that the module has been deprecated and replaced with another module, and point to upgrade procedure.

I'm happy to add maintainers to the project. I've not had time to maintain this since I'm not actually using it in any projects at the moment.

Also, if you all want to work on migrating the functionality to another module, I would be happy to just commit the changes needed for the deprecation.