Add new "llvm:SandboxIR" label to .github/new-prs-labeler.yml (#115965)

As requested in
https://github.com/llvm/llvm-project/pull/115577#issuecomment-2466300749

.github/new-prs-labeler.yml

@@ -587,6 +587,12 @@ llvm:ir:
   - llvm/docs/LangRef.rst
   - llvm/unittests/IR/**
 
+llvm:SandboxIR:
+  - llvm/lib/SandboxIR/**
+  - llvm/include/llvm/SandboxIR/**
+  - llvm/docs/SandboxIR.md
+  - llvm/unittests/SandboxIR/**
+
 llvm:analysis:
   - llvm/lib/Analysis/**
   - llvm/include/llvm/Analysis/**