Adding support for smart_tabs

[florianb]

In 2017 we voted for the introduction of a smart_tabs=true|false property to add support for smart_tabs according to the requirement expressed in this issue but it wasn't implemented yet.

I tried to accelerate that issue by opening a PR on the spec for the final implementation.

I file a new vote - sorry for the noise - to make clear, after 5 years, that this is still the intent of the board and to offer a slight variation as a second option which wasn't considered in 2017:

1. We [...] creat[e] a smart_tabs property, with two options true and false (original option from 2017).
2. We create indent_method property, with now two options smart_tabs and elastic_tabstops for future extension if needed.

If there aren't enough votes i will assume the 2017 vote holds up and go on implementing option 1.

Please vote by commenting below: 1 or 2 only. Voting will be closed at the end of 17.02.2023

Discussion please goes to editorconfig/specification#38

@editorconfig/board-member