Improve documentation/safety of hasExt

[thomasjm]

The hasExt function leaves something to be desired with respect to documentation and type safety. Commit 8288d27 left it in an even worse state, with a reference to the hasExtension function that had been reexported from the now-removed system-filepath dependency. Even worse, that commit introduced a sneaky semantics change where the first arg needs to begin with a dot when before it didn't. It would be good to at least document whether the dot is required, or even better make it foolproof with the type signature somehow.

Shelly.hs/src/Shelly.hs

Line 1499 in 1b6643a

hasExt :: Text -> FilePath -> Bool