github: mark dist/* as generated code (#457)

- per https://docs.github.com/en/repositories/working-with-files/managing-files/customizing-how-changed-files-appear-on-github
  - and https://github.com/github-linguist/linguist/blob/master/docs/overrides.md#generated-code

- should help a good bit with making diffs more condensed by default
  - can still uncollapse generated code, just that default will be collapsed

.gitattributes

@@ -1 +1,2 @@
 * -text
+dist/* linguist-generated