Automatically detect language and generate local.settings.json

If local.settings.json is missing and `func start` is run, attempt to determine the language and create a local.settings.json file with the worker runtime set to the detected language.

If the language can't be detected for a project, print a single warning letting customer know how to resolve it. Currently it's printing a warning multiple times and it's not clear how to resolve (guidance should be to run `func init --` to generate settings file).

#2050 #2831

Start with the `func start` path when `local.settings.json` is missing, then review how `func init --` generates the settings file. The completed work should detect the project language and create the file with the matching worker runtime, or print one actionable warning when detection fails.

Written by the indexing model from the issue text.