Add an editorconfig file with basic rules for editors (#1625)

The configured rules help contributors respect the project styles when
their editor support this (which most editors do either natively or with
a plugin nowadays), even if their own editor configuration has different
defaults.
the configured rules for now are:

- use 2 spaces for indentation
- don't trim trailing whitespaces in HRX files, as some specs rely on
  them

.editorconfig

@@ -0,0 +1,8 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+
+[*.hrx]
+trim_trailing_whitespace = false