Add .editorconfig file

This sets the style for meson (which uses the upstream style, 2 space
indent with no tabs), and sets the tab_width to 8 per the CODING_STYLE
document.

.editorconfig

@@ -0,0 +1,11 @@
+# To use this config on you editor, follow the instructions at:
+# http://editorconfig.org
+
+root = true
+
+[*]
+tab_width = 8
+
+[meson.build,meson_options.txt]
+indent_style = space
+indent_size = 2