Warn when file() matches a collection of files

[bentsherman]

Looking at the standard library functions with an eye towards static type checking, the only problematic function is file() which can return a single file or list of files based on the glob pattern.

Rather than introduce some new function, I think we can transition file() to be properly typed:

1. for now, warn if file() matches a collection of files
2. in the future, change the return type of file() to Path and raise an error if the glob pattern matches zero or multiple files

Note that the glob pattern is still useful for file() when you know it will match only one file but you don't want to spell it out. Raising an error when this assumption is wrong seems like the right approach to me, but of course we should keep it as a warning just for now.

Not sure whether you want to add a test for the warning, but you can see it in action here:

// assumes there are no *.foo files in the launch directory
println file('*.foo')
println file('*.nf')
println files('*.foo')
println files('*.nf')

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[christopher-hakkaart]

Is it appropriate to document files() should be used only for a collection of files at the same time?

[bentsherman]

Agreed, better to describe files() on its own terms

[christopher-hakkaart]

I've added a minor suggestion. Otherwise, the docs look good.