chore: exclude dev files from release archives

- Add .gitattributes with export-ignore for dev files
- Rename assets/ to .wordpress-org/ for consistency
- Update .distignore to exclude .wordpress-org/

Author: GaryJones

.distignore

@@ -1,6 +1,7 @@
 # Directories
 /.git/
 /.github/
+/.wordpress-org/
 /bin/
 /node_modules/
 /tests/

.gitattributes

@@ -0,0 +1,26 @@
+# Exclude from release archives
+/.github/              export-ignore
+/.wordpress-org/       export-ignore
+/bin/                  export-ignore
+/node_modules/         export-ignore
+/tests/                export-ignore
+/vendor/               export-ignore
+/.distignore           export-ignore
+/.editorconfig         export-ignore
+/.gitattributes        export-ignore
+/.gitignore            export-ignore
+/.phpcs.xml.dist       export-ignore
+/.wp-env.json          export-ignore
+/.wp-env.override.json export-ignore
+/composer.json         export-ignore
+/composer.lock         export-ignore
+/package.json          export-ignore
+/package-lock.json     export-ignore
+/phpunit.xml.dist      export-ignore
+
+# Auto detect text files and perform LF normalization
+* text=auto
+
+*.md text
+*.php text
+*.inc text