Allow npm and npx from current Node.js to be referenced using environment variables

Collector supports referencing the node binary from the current Node.js process using the $NODE base call or the ${{env.NODE}} context variable reference. Node.js provides two additional bin scripts, npm and npx. The location of these two scripts should be mapped to the NPM and NPX environment variables. Additionally, it should be possible to reference these locations using the $NPM and $NPX base calls, just like the $NODE base call. For example:

run: $NPM run task

It's currently possible to reference this bin scripts using a relative path:

run: "${{env.NODE}}/../npm run task"

However, this is not an ideal syntax.