editorconfig: Create initial files

Borrow .editorconfig files from the GTK project.  Use the exact
same file for src/gtk since we are still keeping GTK style there
(remove root=true since it's no longer in the root).

The root .editorconfig file is slightly modified since we use
4 space tabs.  Leave tab_width=8 since we do have tabs in the code.

See: https://editorconfig.org/

.editorconfig

@@ -0,0 +1,29 @@
+# SPDX-FileCopyrightText: 2021 The Nautilus Authors
+# SPDX-License-Identifier: CC0-1.0
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+trim_trailing_whitespace = true
+
+[*.[ch]]
+indent_size = 4
+indent_style = space
+insert_final_newline = true
+# For the legacy tabs which still exist in the code:
+tab_width = 8
+
+[*.ui]
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+
+[*.xml]
+indent_size = 2
+indent_style = space
+
+[meson.build]
+indent_size = 2
+indent_style = space

src/gtk/.editorconfig

@@ -0,0 +1,30 @@
+# SPDX-FileCopyrightText: 2021 The GTK Authors
+# SPDX-License-Identifier: CC0-1.0
+
+[*]
+charset = utf-8
+end_of_line = lf
+trim_trailing_whitespace = true
+
+[*.[ch]]
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+# For the legacy tabs which still exist in the code:
+tab_width = 8
+
+[*.ui]
+indent_size = 2
+indent_style = space
+insert_final_newline = true
+
+[*.xml]
+indent_size = 2
+indent_style = space
+
+[meson.build]
+indent_size = 2
+indent_style = space
+
+[*.md]
+max_line_length = 80