Rename environment variable used to enable polling

[mikeharder]

Both dotnet-watch and MVC use environment variables to enable polling. Currently, dotnet-watch uses USE_POLLING_FILE_WATCHER, while MVC uses ASPNETCORE_POLL_FOR_FILE_CHANGES. To simplify things for customers, including the Dockerfile generated by Docker for VS Tooling, we would like to use the same variable for both dotnet-watch and MVC, and name it accordingly.

Related: aspnet/Mvc#4696

[muratg]

@Eilon Can we agree on a name? @victorhurdugaci will do the change both here and on aspnet/Mvc#4696

cc @pranavkm

[mikeharder]

Existing names:

USE_POLLING_FILE_WATCHER (dotnet-watch)
ASPNETCORE_POLL_FOR_FILE_CHANGES (mvc)

Proposals for unified name:

WATCHER_POLL_FOR_FILE_CHANGES
WATCHER_USE_POLLING
NETCORE_POLL_FOR_FILE_CHANGES

[muratg]

DOTNET_USE_POLLING_FILE_WATCHER?

[Eilon]

Yeah I dig that name.

[Eilon]

That is, the name that @muratg suggested.

[muratg]

@victorhurdugaci please update it both here and in MVC.