[FEATURE] List all available languages

[Nala Ginrut]

As I talked with Andy, we may add a feature to list all available
languages. And if users add a new language to the %load-path, they may
see it listed out. Besides, this feature could be used in 'guild
compile' to detect all the language available.

Attached is a proc named 'get-all-available-languages', it'll scan all %
load-path and list the union of the languages available. 

Please review it.
If accepted, I'll format a patch into (ice-9 command-line).

Thanks!

lang.scm
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