[Feature Request] func start should be able to leverage dotnet watch

Currently, intensive local development with `func start` is tricky and time-consuming, since there is no live-reload behavior, not event with `dotnet watch run`, and `CTRL+C` plus `func start --build` is still slow (beta 35)

Start at the `func start` command and investigate how it currently builds and launches local Functions projects. Determine how `dotnet watch` could provide live reload without the existing CTRL+C and `--build` cycle. Done means intensive local development with `func start` has a working live-reload path.

Written by the indexing model from the issue text.