[Custom Elements with Button Activation Behaviors] Add minimal decomposed alternative (#1162)

Added ElementInternals feature decomposition approach
- Introduction
- Key characteristics and differences vs main proposal
- Example
- Supported attributes, properties and events
- Disadvantages, example
- Notes on accessibility + issue in ARIA WG and consensus 

Throwing exceptions
- Throw DOM Exceptions when properties are not available

Other changes
- Note on interest for
- Updates to default type
- Future work

Some nits
- Spelling
- Extra spaces
- Uniform general structure for better readability

Author: anaskim