When a new keymap is defined, the user takes an existing one, and
creates a new one based on the old ones (i.e. inheritance).
I want to be able to do the same with file types.
An example is HTA files. They are essentially HTML files with special
HTA tags. Functionally, I want IntelliJ to give me the same features as
HTML files, plus to recognize the HTA tags.
Currently, in IntelliJ this is impossible.
HTA files should be defined as "this is based on HTML files plus the HTA