Validate folder is WPILib project

[PeterJohnson]

It's relatively common for teams to open the wrong folder (e.g. one level higher than the project) and then be confused when build/deploy commands result in a "gradlew" not found error. It would be helpful for usability if WPILib provided a visual indicator of whether the current folder is a valid project when open and/or popped up an error messagebox on build/deploy command explaining the issue rather than requiring the user to decipher the command line error.

[ThadHouse]

We actually do.

https://github.com/wpilibsuite/vscode-wpilib/blob/main/vscode-wpilib/src/extension.ts#L365

It's done this forever. People just close the warning.

[ThadHouse]

I guess we could also make this so it unconditionally pops up on any of the gradle commands. Instead of just on start.