chore: add LICENSE

[kopecs]

We previously did not distribute a specific license file with this repository since it is included in semgrep/semgrep as a submodule, but for clarity as to how this specific set of files is licensed (and because users may obtain them separately) let's add a copy of the license here.

Closes semgrep/semgrep#10850.

• I ran make setup && make to update the generated code after editing a .atd file (TODO: have a CI check)
• I made sure we're still backward compatible with old versions of the CLI.
  For example, the Semgrep backend need to still be able to consume data
  generated by Semgrep 1.50.0.
  See https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades
  Note that the types related to the semgrep-core JSON output or the
  semgrep-core RPC do not need to be backward compatible!

[github-actions]

Backwards compatibility summary:

Checking backward compatibility of semgrep_output_v1.atd against past version v1.100.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.101.0
Skipping v1.102.0 because commit 1c82453e89e0b569630e48ddde015e201df0e5f9 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.103.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.104.0
Skipping v1.106.0 because commit 5e0c767ec323f3f2356d3bf8dbdf7c7836497d8a has already been checked
Skipping v1.107.0 because commit 5e0c767ec323f3f2356d3bf8dbdf7c7836497d8a has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.108.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.75.0
Skipping v1.76.0 because commit 9102031608aa4154e1c37f557550ec4eabc8780c has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.77.0
Skipping v1.78.0 because commit dcb5d77b420ddee61f58aadd3c2c7aef38778154 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.79.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.80.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.81.0
Skipping v1.82.0 because commit 9e0f3bec26b07b4fb6753a32cb75277f45f2572c has already been checked
Skipping v1.83.0 because commit 9e0f3bec26b07b4fb6753a32cb75277f45f2572c has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.84.0
Skipping v1.84.1 because commit 3daef49297ada205359cc1d2996354c94b628b0d has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.85.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.86.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.87.0
Skipping v1.88.0 because commit 512c0bd97db59c48a5705b2741662a338776e438 has already been checked
Skipping v1.89.0 because commit 512c0bd97db59c48a5705b2741662a338776e438 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.90.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.91.0
Skipping v1.92.0 because commit 2351c5e528cb7430422208dc66707894c066b508 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.93.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.94.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.95.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.96.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.97.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.98.0
Skipping v1.99.0 because commit 60809032a2e39742f42910d46b3e5dd305b8b8cf has already been checked