add variable stop, start and transform methods

[Aylur]

this introdues these to ags.Variable

const variable = ags.Variable(null, {
    // can be a string or string[] like before
    listen: 'path-to-script',
    listen: ['bash', '-c', 'some-bash-command'],
    poll: [5000, 'some-command-or-script'],
    poll: [5000, ['bash', '-c', 'some-command']],

    // now you can pass a transform method
    listen: ['path-to-script', out => JSON.parse(out)],
    listen: [['bash', '-c', 'some-bash-command'], out => JSON.parse(out)],
    poll: [5000, 'some-command-or-script', out => JSON.parse(out)],
    poll: [5000, ['bash', '-c', 'some-command'], out => JSON.parse(out)]
});

// temporarily stop them
variable.stopListen(); // this kills the subprocess
variable.stopPoll();

// start them
variable.startListen(); // launches the subprocess again 
variable.startPoll();

// get if they are active
print(variable.isListening);
print(variable.isPolling);

// cpu usage for example
const cpu = ags.Variable(0, {
    poll: [5000, 'top -b -n 1', out => out.split('\n')
        .find(line => line.includes('Cpu(s)'))
        .split(/\s+/)[1]],
});