CONTRACTS: add loop contract mode enum and string conversion functions

[remi-delmas-3000]

Used by loop contracts and error messages.

Extracted from #7541, can only be tested once all features are in place.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4413298) 78.51% compared to head (d8a78cc) 78.51%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7628   +/-   ##
========================================
  Coverage    78.51%   78.51%           
========================================
  Files         1674     1674           
  Lines       191954   191954           
========================================
  Hits        150721   150721           
  Misses       41233    41233           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[tautschnig]

Consider comments at your leisure.

[qinheping]

LGTM