Add .editorconfig

[abettermap]

Description

General way to promote tab/space consistency and other language-generic rules: https://editorconfig.org/

Related items

Jira CSR-174

Resolution

• [ ] Add to docs (CONTRIBUTING.md most likely)
• [ ] Commit this:

# top-most EditorConfig file
root = true

# Project-wide settings (all files)
[*]
end_of_line = lf
insert_final_newline = true
indent_style = space
indent_size = 2

# TOC bug in GitLab Markdown parsing:
# github.com/DavidAnson/markdownlint/blob/master/doc/Rules.md#md007---unordered-list-indentation
[*.md]
indent_size = 4