resourceloader: Create mediawiki.base.js and move mw.inspect to it

resourceloader: Create mediawiki.base.js and move mw.inspect to it

As a first step to splitting the 'mediawiki' module, start by
splitting the file. The two files (like the other files in the
same directory) are still concatenated and run at the same time.

The only difference being that the private variables aren't available,
and it forces us to think about not being able to call the methods
during initialisation given the base file will execute after the
main one, making its methods not yet available during the split
time between the two.

From the perspective of regular modules, this changes nothing
as it will still load as one atomic unit defining the same public
API as before.

Bug: T192623
Change-Id: I552ae02e49c4d30070e166a504f454e334e8e75e