resourceloader: Add forward-compat to startup.js for I1c995a9572d
This prepares startup.js to support the declarative format
for transporting values for mw.config, mw.loader.state, and
mw.loader.load.
These are currently applied as executable code in <head>, but after
I1c995a9572d these will instead be declarative allowing them
to be set without a closure and consolidating control over the
startup flow to startup.js.
We'll still have RLQ for two reasons: 1) compat with cached HTML,
and 2) for the use case of streaming load.php-like responses
as part of the HTML payload (e.g private modules, previews,
inline snippets).
Change-Id: I76a2c8d52fa782cd09d48d99b5fbb7be35e2b9a8