ci: add stale theme pull request closer action push (#2067)

This commit adds the `stale-theme-pr-closer` closer action. This action
is used to close theme pull requests with an `invalid` label that has
been stale for a given number of `STALE_DAYS`.

package.json

@@ -11,6 +11,7 @@
     "test:e2e": "node --experimental-vm-modules node_modules/jest/bin/jest.js --config jest.e2e.config.js",
     "theme-readme-gen": "node scripts/generate-theme-doc",
     "preview-theme": "node scripts/preview-theme",
+    "close-stale-theme-prs": "node scripts/close-stale-theme-prs",
     "generate-langs-json": "node scripts/generate-langs-json",
     "format": "./node_modules/.bin/prettier --write .",
     "format:check": "./node_modules/.bin/prettier --check ."