Add diagnostic for pluscal labels inserted by translation

[hwayne]

The PlusCal translator has a -reportLabels flag. When set, it will automatically add missing labels to translations and log that in the output:

pcal.trans Version 1.11 of 31 December 2020
The following label was added:
  Lbl_1 at line 16, column 5
Parsing completed.
Translation completed.

Since it's logged, we might as well tell the user about it! This PR adds an information diagnostic. Demo:

Questions:

• Should it be a warning instead of information?
• In the long run should the pluscal parser use some kind of state machine?