Add .editorconfig file

This informs a broad variety of text editors of some of the formatting
conventions of the filament project. It may be possible to replicate some of the
more specific formatting conventions like brace placement in a future commit.

.editorconfig

@@ -0,0 +1,11 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+
+[*.{c,cpp,h,inc,kt,java,js,md}]
+indent_style = space
+indent_size = 4
+max_line_length = 100