editorconfig: add initial file/config

editorconfig configuration files hold editor style hints, and
is supported by many popular editors. See https://editorconfig.org/ .

The vast majority of the mpv source/text files are already styled as
4 space indentation, trailing newline at EOF, and UTF-8 encoding.

This commit adds a single .editorconfig root file which applies these
rules to all files using the glob "[*]".

If it turns out to be too inclusive then we can narrow it down later.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

.editorconfig

@@ -0,0 +1,10 @@
+# To use this config on you editor, follow the instructions at:
+# http://editorconfig.org
+
+root = true
+
+[*]
+charset = utf-8
+insert_final_newline = true
+indent_style = space
+indent_size = 4