Allow open-ended layer zooms

[pnorman]

:zoom=>a..b translates to {"minzoom":a, "maxzoom":b}. This works well for zooms in the "middle" of the ranges, but when dealing with high zooms it is useful to have an open-ended zoom where only one of the two properties is specified. This is not currently possible.

[gravitystorm]

I agree, this is very useful and I've used it in my own stylesheets. There's no need for us to have to type X..24 all the time, when X.. will do, and has a clearer meaning.

I've implemented it, along with tests, at gravitystorm@6589722

Ruby introduced Endless Range in 2.6 and Beginless in 2.7 so we can implement them here.

When it comes to a PR, would you like just the code, or should I make a separate PR for adding the test suite first? I certainly felt more comfortable with a minimal test suite before I started poking around too much with the glug internals 😄

[systemed]

Very happy to have PRs for test suite and for endless/beginless ranges! No great preference as to which comes first.