Extend PageMatcher with Environment keyword

[CathrinePaulsen]

Fixes #9612

This fix allows environment to be used as a keyword for cascade._target.
It can be used to apply cascade options to the target in only certain build environments, for example to specify in which environment(s) certain pages should not be rendered.

The documentation has been updated with the new keyword.

[bep]

0K, this PR looks mostly good, but we don't want to add the Environment() everywhere -- the Page interface is big as it is.

So I suggest that you in the page matcher do something ala:

if m.Environment != "" {
		g, err := glob.GetGlob(m.Environment)
		if err == nil && !g.Match(p.Site().Hugo().Environment()) {
			return false
		}
	}

[CathrinePaulsen]

Thank you for the feedback! That makes sense, I'll change it.

The reason for the current implementation was to make extending the PageMatcher test as easy as possible, since otherwise it will panic when calling p.Site() since it's not implemented for testPage.

How do you suggest I change the PageMatcher test? Should I implement testPage.Site() or is there a better way?

[bep]

Yea, I see your point. Ultimately we may want to split it (e.g. having both a MatchesSite and a MatchesPage method or something), but even then you will have the same test challenge.

There is a testSite implementation in there somewhere, so I suggest you just expand on that. It makes for a little more work, but certainly a better end game.

[CathrinePaulsen]

I think the most recent commit should address the requested change. Let me know what you think!

[bep]

Yea, now it looks good, thanks!

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