JSIL: create messaget with a message handler

[tautschnig]

Apply RAII and avoid a deprecated messaget constructor.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (92e3c0f) 79.08% compared to head (e13dfe4) 79.08%.

Files	Patch %	Lines
src/jsil/jsil_entry_point.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8125   +/-   ##
========================================
  Coverage    79.08%   79.08%           
========================================
  Files         1697     1697           
  Lines       196493   196492    -1     
========================================
+ Hits        155395   155398    +3     
+ Misses       41098    41094    -4     

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