Add .editorconfig file (ECP-WarpX#1261)

This is a widely supported format for basic IDE/editor settings that
we care about. Will most of the time be auto-recognized by tools,
others have plugins.

https://editorconfig.org

.editorconfig

@@ -0,0 +1,32 @@
+# http://EditorConfig.org
+#
+# precedence of rules is bottom to top
+
+# this is the top-most EditorConfig file
+root = true
+
+
+[*]
+# 4 space indentation
+indent_style = space
+indent_size = 4
+
+# no end of line whitespaces
+trim_trailing_whitespace = true
+
+# unix-style newlines
+end_of_line = lf
+
+# newline ending in files
+insert_final_newline = true
+
+
+[*.md]
+# two end of line whitespaces are newlines without a paragraph
+trim_trailing_whitespace = false
+
+
+[Makefile]
+# TABs are part of its syntax
+indent_style = tab
+indent_size = unset