settings reader to replace torchbear.scl and app.scl

[naturallymitchell]

this will takeover https://github.com/foundpatterns/lighttouch/blob/8d3a510a15326890b722de549f5a5fdf96a26f08/config.lua

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should support overrides

related:

• set env settings variables from app-settings and cwd-settings files jazz#241
• trace() app name and checksumdir jazz#242
• better app settings for app's apps jazz#254
• apps using torchbear interpreter should read torchbear.scl config file from "app-dir" jazz#196
• https://github.com/foundpatterns/torchbear/search?q=current_dir&unscoped_q=current_dir
• Bug fix: prepend app path to torchbear.scl jazz#205

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if contentdb supports outlines, then each torchbear app could just include an expected outline schema
https://github.com/foundpatterns/contentdb/issues/21