Remove JSIL front-end

[tautschnig]

We do not have any tests and JSIL is not being developed any further at https://vtss.doc.ic.ac.uk/research/javascript.html.

• 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.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a 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

All modified and coverable lines are covered by tests ✅

Comparison is base (fd28cb2) 79.09% compared to head (67be193) 79.69%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8158      +/-   ##
===========================================
+ Coverage    79.09%   79.69%   +0.59%     
===========================================
  Files         1695     1680      -15     
  Lines       196589   195104    -1485     
===========================================
- Hits        155493   155479      -14     
+ Misses       41096    39625    -1471     

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[martin-cs]

Approve in the sense that I believe this is technically correct.

I have emailed Philippa to see if she has any thoughts on this but given POPL starts in a few days I think it might take a little while to hear back.

[tautschnig]

Approve in the sense that I believe this is technically correct.

I have emailed Philippa to see if she has any thoughts on this but given POPL starts in a few days I think it might take a little while to hear back.

I firmly believe in version control and am trying to keep commits clean. We should be able to revert quite easily if the need arises.