Cut-paste the configFuncSettingsFrom() function and its unit tests out
of settings.js and into its own config-func-settings-from.js file.

This is because in a future refactoring index.js isn't going to call
configFuncSettingsFrom() anymore, only settings.js will call it, which
would make configFuncSettingsFrom() a private helper function of
settings.js, but I don't want to lose configFuncSettingsFrom()'s unit
tests so I'm moving it into its own file from where settings.js (in the
future, index.js currently) can import and use it. This way
configFuncSettingsFrom() remains part of the public API of its
containing module and can have unit tests.