Adopt vscode.env.shell

This new API was added since terminal.integrated.shell.* now defaults to null. https://github.com/microsoft/vscode/issues/75091

I recommend adopting env.shell and use that when it’s available and remove the default shell detection logic added in https://github.com/microsoft/vscode-python/pull/5886 in a few months and instead fallback to very basic detection (cmd, bash || sh)

[Tyriar]

@DonJayamanne remember to include a fallback:

public get shell(): string {
    if ('env' in vscode) {
        // tslint:disable-next-line: no-any
        return (vscode.env as any).shell;
    }

    return isWindows ? 'cmd.exe' : process.env.SHELL;
}

[Tyriar]

@DonJayamanne please adopt this before it goes stable, basically the only point of the proposed period is for you to report issues with it.