Add an .editorconfig file to repository

This commit tries to use an editorconfig file to ensure the same
settings across editors while contributing to the vim repository.

The rules are based of the guidelines defined in
`runtime/doc/develop.txt`.

Signed-off-by: Luca Saccarola <github.e41mv@aleeas.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

.editorconfig

@@ -0,0 +1,21 @@
+# https://spec.editorconfig.org/#supported-pairs
+root = true
+
+[*]
+indent_style = tab
+tab_width = 8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.{c,h,proto}]
+indent_size = 4
+
+[*.{md,yml,sh,bat}]
+# This will become the default after we migrate the codebase
+indent_style = space
+indent_size = 2
+
+[*.md]
+# Markdown uses trailing whitespaces to do an hard line break
+# https://spec.commonmark.org/0.31.2/#hard-line-breaks
+trim_trailing_whitespace = false