chore: add .editorconfig and .gitattributes, store validate.py with LF

.editorconfig is the same as TechEngine's: UTF-8, LF, final newline,
4 spaces for Python and 2 for YAML, JSON, Astro, Markdown and mjs.
.gitattributes sets `* text=auto eol=lf`, so a Windows checkout no
longer turns files into CRLF.

app/validate.py was the only tracked text file stored with CRLF (all of
data/ is LF already). It is renormalised here: every line shows as
changed, but `git diff --ignore-cr-at-eol` against the parent is empty.

Author: Seungpyo1007

.editorconfig

@@ -0,0 +1,18 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.py]
+indent_style = space
+indent_size = 4
+
+[*.{yml,yaml,json,astro,md,mjs}]
+indent_style = space
+indent_size = 2
+
+[*.md]
+trim_trailing_whitespace = false

.gitattributes

@@ -0,0 +1,3 @@
+# Every tracked text file is stored with LF; keep working trees the same on
+# Windows and macOS.
+* text=auto eol=lf