Fix storing context vars value as string

[kolumb]

I was using variables in duration expression and noticed that it's concatenating value as strings.
For example if you change duration of Hop filter to "duration": "interval + 2" it calculates it as 0.852 instead of 2.85 here:

const duration = Math.min(run_expr(program.duration, context), 60);
console.log({duration})

[rexim]

@kolumb good catch! Thank you very much! 👍 The reason why TypeScript didn't help in here is because of Tag being any which we gonna fix in #73