misc: init .editorconfig

TL;DR: Improved developer productivity for many contributors

Add a shared .editorconfig so contributors get lightweight,
editor-native hints while writing code instead of only discovering
formatting issues later in style checks.

This is advisory, but useful: editors can already guide indentation,
whitespace, line endings, and final newlines as you type. They can also
show a visual guide at the 80-character line width, which is one of the
main motivations here. Most LLM-generated code and contributions over
the past year have already conventionally followed the 80-character
limit, so this makes the expected style visible and consistent for
everyone.

.editorconfig is a decade-old standard, and virtually every editor or
IDE supports it.

Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>

.editorconfig

@@ -0,0 +1,25 @@
+# https://editorconfig.org/
+#
+# Hints for editors to assist with correct formatting as you type.
+root = true
+
+# Unix-style newlines with a newline ending every file
+[*]
+charset = utf-8
+indent_size = 4
+end_of_line = lf
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+# Recommendation, not enforced.
+max_line_length = 80
+
+[Makefile]
+indent_style = tab
+
+# Inherited as default
+# [*.sh]
+# indent_size = 4
+
+[{Cargo.lock,*.md,*.toml,*.yml,*.yaml}]
+indent_size = 2