Simple hello world for pkg, useful for smoke testing whether compiled pkg binaries run

1. Copy built binaries locally somewhere, with current pkg-fetch major.minor as last directory in the path. E.g. C:\Temp\pkg-cache\v3.4
2. Rename node-* files to built-* to make pkg think these binaries were built locally
3. Update the local patch file in ./patches/pkg-fetch*.patch to enable the version you are testing. Rerun yarn, possibly removing node_modules if necessary, e.g. if the patch doesn't apply. E.g:

index c3bb657..4356f83 100644
--- a/node_modules/pkg-fetch/patches/patches.json
+++ b/node_modules/pkg-fetch/patches/patches.json
@@ -1,4 +1,5 @@
{
+  "v18.15.0": ["foo.bar"],
  "v18.5.0": ["node.v18.5.0.cpp.patch"],
  "v16.16.0": ["node.v16.16.0.cpp.patch"],
  "v14.20.0": ["node.v14.20.0.cpp.patch"],

4. Set/export PKG_CACHE_PATH=<parent directory of v3.4>

E.g. set PKG_CACHE_PATH=C:\Temp\pkg-cache in the example in 1. on Windows.

5. Run yarn worker-demo --target node18.15.0 --build

note: This will try and "build" 18.15.0, but a built binary already exists in the PKG_CACHE_PATH so it will reuse it. We are tricking pkg into using the binary we want to test, and bypassing the sha256 hash check that normally comes when we download fetched-* binaries to the local cache.