Ciao,
this patch replaces the dependency on {js,css,html}-ts-mode with the 
dependency on ‘mhtml-ts-mode’. 
Code maintenance should now be easier. 
Additional benefits are:
1. Imenu now exposes symbols for all of all languages,
2. navigation now works correctly for all languages,
3. outline works for all languages.
Additional new features are:
1. indentation of PHP in mixed buffers with HTML now works much better and 
allows three different behaviors, an option allows you to choose the behavior.
2. a new feature shows where PHP ini files are both locally and remotely, if 
the buffer is associated with a remote PHP file.

The patch is a little long, but I hope it's okay.
Thank you.

Vincenzo