`overlay use` for core

Use definitions from a module as an overlay.

Signature

> overlay use (name) (as) --prefix --reload

name: Module name to use overlay for
as: as keyword followed by a new name
--prefix (-p): Prepend module name to the imported commands and aliases
--reload (-r): If the overlay already exists, reload its definitions and environment.

This command is a parser keyword. For details, check: https://www.nushell.sh/book/thinking_in_nu.html

Create an overlay from a module

> module spam { export def foo [] { "foo" } }
    overlay use spam
    foo

Create an overlay from a module and rename it

> module spam { export def foo [] { "foo" } }
    overlay use spam as spam_new
    foo

Create an overlay with a prefix

> 'export def foo { "foo" }'
    overlay use --prefix spam
    spam foo

Create an overlay from a file

> 'export-env { let-env FOO = "foo" }' | save spam.nu
    overlay use spam.nu
    $env.FOO