Add a .editorconfig for this project (nikic#162)

* [skip ci] Add a .editorconfig for this project

Add default indentation choice for file types where it currently isn't a mix.

* [skip ci] Suggest spaces for phpt tests

The majority of files use spaces

Author: TysonAndre

.editorconfig

@@ -0,0 +1,17 @@
+# https://editorconfig.org/
+root = true
+# 4 space indentation (no size specified)
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 4
+trim_trailing_whitespace = true
+insert_final_newline = true
+[*.{c,h}]
+indent_style = tab
+[*.xml]
+indent_size = 1
+indent_style = space
+[*.{md,php,phpt}]
+indent_size = 4
+indent_style = space