Use scopes to direct logs to correct file

Today the FileLogger (what you see in the portal when invoking functions) figures out the correct file to write to based on the `Function.{FunctionName}.User` standard category that we use. However, we should be able to look at the scope as well to see if there's a function name. If so, write there.

Start by locating FileLogger and tracing how the Function.{FunctionName}.User category selects the output file. Then inspect how logging scopes expose a function name and verify that scoped function names direct logs to the correct file while the existing category behavior remains intact.

Written by the indexing model from the issue text.