This lets various supporting editors know what indentation style should
be used for files in the repo. See https://editorconfig.org/
Coding style for Lua files is described in `doc/coding_style.md`
The 3-space indentation in `configure` comes from its LuaRocks
inheritance.
`doc/doap.xml` is normalized with `xmllint` which spits out 2-space
indentation.
