httpcaddyfile: Add {vars.*} placeholder shortcut, reverse vars sort order

[francislavoie]

I'm yoinking the placeholder from my #4657 PR because I think we should get this in ASAP for v2.5.0 along with the new vars directive.

Also, during testing while writing the docs, I realized we should reverse the vars directive sort order based on matchers; since this is a directive which overwrites existing values set by other previously run directives, we actually want the least-specific matcher to run first, and most-specific last, so that the most-specific is the last one to overwrites the value.

For example, with this config:

vars foo bar
vars /path* foo baz

respond {vars.foo}

Before this change, all requests would return bar, because the least specific matcher would run last, overwriting any more specific matcher. After the change, /path* requests will correctly return baz.

[mholt]

Thanks for this. Seems to be a good change, has a test, and is minimally invasive. Will merge this for 2.5.