Create $(let ...) providing lexically scoped variables

Create $(let ...) providing lexically scoped variables

Add a new function $(let ...) which allows lexically scoped variables.

* NEWS: Add information on this feature.
* doc/make.texi (Let Function): Document the 'let' function.
* src/function.c (func_let): Create the 'let' built-in function.
* tests/scripts/functions/let: Test the 'let' built-in function.