Make FileWatching.Pidfile public (#61208)

As the documentation mentions it explicitly and e.g. suggest using
[FileWatching.Pidfile.mkpidfile](https://docs.julialang.org/en/v1.12/stdlib/FileWatching/#FileWatching.Pidfile.mkpidlock)
among other things.

Without this,
[ExplicitImports](https://github.com/JuliaTesting/ExplicitImports.jl)
complains (CC @ericphanson)

Author: fingolfin

stdlib/FileWatching/src/pidfile.jl

@@ -1,3 +1,6 @@
+"""
+A simple utility tool for creating advisory pidfiles (lock files).
+"""
 module Pidfile
 
 
@@ -372,3 +375,5 @@ function Base.close(lock::LockMonitor)
 end
 
 end # module
+
+public Pidfile