Proposal: Splitting modules into three types of modules - low-level, widgets, and controls

(I've spoken to @timbl about this earlier, and we think this is a good idea, but not written it down here, so thought I would create an issue to open it up for discussion.)

Currently the API of Solid UI is split up into modules that usually have a mix of low- and high-level functionality. To clear up these we propose a convention that splits modules into three types: low-level, widgets, and controls. The idea is that the types handle increasingly complex methods in terms of the DOM elements that are returned.

1. The low-level methods do not deal with DOM elements at all, but deal with data manipulation and utility functions that are useful when working with Solid.
2. Widgets return simple DOM elements, such as buttons and other elements that are meant to be woven into a larger compositions.
3. Controls are complete DOM elements in the sense that they can work stand-alone and can expose complex interactions.

The modules would follow a name convention given their type, e.g. low-level modules would simply be named the functionality they expose, widget modules would be postfixed Widget, and control modules would be postfixed Control. This way a range of functionality that all reflect the same type of functionality could all start with the same name, but the two latter would expose higher complexity, e.g. acl, aclWidget, and aclControl.

We also propose that existing API endpoints that would be deprecated as part of this restructure to be deprecated slowly, giving people who depend on the existing API ample time to update their dependencies.

Start by reviewing the current Solid UI API modules and cataloging which endpoints mix low-level utilities, widgets, and controls. Compare the existing names and DOM responsibilities with the proposed conventions, then document the migration and deprecation scope. Done means the module boundaries, naming changes, and gradual deprecation plan are agreed.