code lens: add a 'run' code lens for main functions

[stamblerre]

See the original feature request: microsoft/vscode-go#3074.

[zhylmzr]

This feature is very helpful for single file and implementation inspiration.

[hyangah]

Should it run in the single file mode, or run the package?

[AntBlo]

I'd find this useful for cmd/*.go files. I believe I've seen tests that have run/debug code lenses.
Having the same feature for main seem like a good idea to me.

[hyangah]

@vuon9 Thanks for the PR!

I see the current implementation is similar to run test codelens,
creates a dedicated output channel called "Go Main" and forwards go run . output to stdout/stderr.
I now wonder if that's too limiting for a run test though.

What do you think about utilizing the task provider (https://code.visualstudio.com/api/extension-guides/task-provider)?
I guess that may allow access to stdio (including `stdin) and also open up the possibility of users can customize further to add environment or arguments.