Add .gitattributes to mark generated files for linguist

For example, you might generate code, or create a file that combines multiple files into a single file. When you include these files in the repository, they might be included in the PR, which can be distracting. So, by default, the generated files will not appear in the diff.

refs: https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github

Author: ydah

.gitattributes

@@ -0,0 +1,2 @@
+lib/lrama/parser.rb linguist-generated=true
+sig/** linguist-generated=true