Add default codeowners to /src/symtab2gb/ and /src/json-symtab-language/

[thomasspriggs]

The existing lines in the CODEOWNERS file overrides the default code owners, leaving Martin as the sole code owner. Adding the default code owners to the list should make it more straight forward to maintain these areas.

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (d01fa33) 79.09% compared to head (3b5096a) 79.09%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8087   +/-   ##
========================================
  Coverage    79.09%   79.09%           
========================================
  Files         1701     1701           
  Lines       196625   196625           
========================================
  Hits        155527   155527           
  Misses       41098    41098           

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