Improve support for Scheme R6RS and R7RS libraries (bug#54704)

Improve support for Scheme R6RS and R7RS libraries (bug#54704)

* etc/NEWS (Scheme mode): Document improved file-type auto-detection
and Imenu support for R6RS and R7RS Scheme libraries.
* lisp/files.el (auto-mode-alist): Associate the '.sls' (R6RS Scheme
Library Source) and '.sld' (R7RS Scheme Library Definition) file name
extensions with the Scheme mode.
* lisp/progmodes/scheme.el (scheme-imenu-generic-expression): Make
Imenu recognize the members nested (and so indented) inside of
'library' (R6RS) or 'define-library' (R7RS) forms.