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

As requested in
llvm#115577 (comment)

Author: slackito

.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/**