misc: add .editorconfig file

[Mopolo]

No description provided.

[szepeviktor]

It could be checked :) in CI

        -
            name: "Checkout repository"
            uses: "actions/checkout@v3"
        -
            name: "Check EditorConfig configuration"
            run: "test -f .editorconfig"
        -
            name: "Check adherence to EditorConfig"
            uses: "greut/eclint-action@v0"

[romm]

I'm not against it, but currently we have an issue with Markdown files. Take the following:

1. Scalar types will only accept values that strictly match their type.
2. Objects will only accept values that can be mapped to their constructor
   parameters, unneeded array entries will cause a mapping error.
3. Arrays and lists will only accept values that strictly match their subtypes
   and structure.

Lines 3 and 5 report errors, because the "indentation" is considered as wrong with 3 spaces. There are currently ~40 similar errors in the doc (basically every multilined-lists) and I currently have no idea how to fix that.

Any idea?