changed true/false to "true"/ "false" as boolean is still not supported

[NeveIsa]

The following example results in error.

def is_even(number):
  if number % 2 == 0:
    return true
  else:
    return false

def main():
  return is_even(10)

╰─$ bend run filename.bend
Errors:
In definition 'is_even':
Unbound variable 'false'.
Unbound variable 'true'.

[geekyayush]

Thanks for the PR. I will have a look at it.