Support for `pnpm` packages

Since `dotly` already supports `volta` and `npm`, `pnpm` would be a great addition IMO. I could develop it myself, should be something like:

```sh
# Dump
pnpm ls -g --json | jq -r '.[0].dependencies | keys[]' >"$PNPM_DUMP_FILE_PATH"
# Import
xargs -I_ pnpm install -g "_" <"$PNPM_DUMP_FILE_PATH"
```  

