Add `library` configuration to `stdlib-fpm`

[perazz]

Motivation

Users may want to build stdlib using fpm, and then use it as a static library in other projects.
To do so, I suggest to add a [library] configuration in the fpm.toml manifest.

Prior Art

See also: #640
#781

Additional Information

No response

[moriglia]

If you don't mind, I would like to add the [install] section to the fpm.toml file.

This has the effect that if I type fpm install [--profile release] on the root of stdlib, it will copy the compiled library to ~/.local/lib/ and the .mod/.smod files to ~/.local/include/

I will prepare a PR in a moment, for you to review.