Add .editorconfig

This file codifies the preferred editor configuration when working
on V8. Including it in the repository makes it easier for contributors
to follow the existing coding style.

See http://editorconfig.org/ for more information.

Change-Id: Iea69c29fc0d88e0fd085e4b3ffc46697d0cd1202
Reviewed-on: https://chromium-review.googlesource.com/591427Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46973}

.editorconfig

+root = true
+
+[*]
+charset = utf-8
+indent_style = space
+indent_size = 2
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true