Add op_node_unstable_os_uptime to allow for node interop

Editor assessment

Mostly fine, with two soft spots. The capital 'Add' breaks the lowercase-start convention, and the rationale 'to allow for node interop' is oddly generic in a node-compat commit — the specific motivation (Node's os.uptime() was missing, breaking some npm package) is what makes such messages valuable. Adding a new op is also arguably feat rather than fix, since it creates capability that didn't exist; as fix it implies the op's absence was a defect, which is defensible for interop parity but worth stating. The op name itself is well chosen for greppability. A one-line body would have settled all of this.