Other: Add .editorconfigs

Add `.editorconfig`s, to help maintain a consistent coding style.
Also apply the new style to existing files.

.editorconfig

@@ -0,0 +1,15 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+charset = utf-8
+
+[*.{js,json}]
+indent_size = 4
+trim_trailing_whitespace = true
+
+[*.md]
+indent_size = 4
+trim_trailing_whitespace = false

docs/.editorconfig

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+# Unset rules for `.js` files in this directory, since they might come from
+# external sources and therefore shouldn't be affected by this repos
+# `.editorconfig`.
+[*.js]
+indent_size = unset
+trim_trailing_whitespace = unset