determine correct way to express SpiderNode dependency on static:js

positron/webidl/moz.build contains this declaration:

```
# Declare a dependency on static libjs in an attempt to force it to get built
# before compiling and linking SpiderNode.
# TODO: figure out a better way to express this dependency (and SpiderNode's
# dependency on icudata.o, for that matter).
USE_LIBS += ['static:js']
```

So we should do what it says.
