Simplify rule_schema_v2.atd, use less variant

[aryx]

This better match the syntax.
The goal is not to redo Rule.ml and have clean variant types!
The goal is to specify the syntax.

test plan:
see related PR in semgrep

• 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 1.17.0.
  See https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades

[github-actions]

Backwards compatability summary:

Checking backward compatibility of semgrep_output_v1.atd against past version v1.16.0
Skipping v1.17.0 because commit efc8e5c0076c8d460c0f802bc1fb2732490d0103 has already been checked
Skipping v1.17.1 because commit efc8e5c0076c8d460c0f802bc1fb2732490d0103 has already been checked
Skipping v1.18.0 because commit efc8e5c0076c8d460c0f802bc1fb2732490d0103 has already been checked
Skipping v1.19.0 because commit efc8e5c0076c8d460c0f802bc1fb2732490d0103 has already been checked
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Skipping v1.21.0 because commit efc8e5c0076c8d460c0f802bc1fb2732490d0103 has already been checked
Skipping v1.22.0 because commit efc8e5c0076c8d460c0f802bc1fb2732490d0103 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.23.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.24.0
Skipping v1.24.1 because commit 213f67abea73546ca6111e1bbf0ef96aa917c940 has already been checked
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Skipping v1.27.0 because commit 213f67abea73546ca6111e1bbf0ef96aa917c940 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.28.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.29.0
Skipping v1.30.0 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
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Skipping v1.31.1 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
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Skipping v1.32.0 because commit 78720c795cd5a186f5102c87125ef876c6435a0c has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.33.0
Skipping v1.33.1 because commit 8849e56ddb0977e38a120a6cfbd1c396eb6fa15e has already been checked
Skipping v1.33.2 because commit 8849e56ddb0977e38a120a6cfbd1c396eb6fa15e has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.34.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.37.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.38.0
Skipping v1.38.1 because commit fd294683e7369cabf63738febeaba8a22c925187 has already been checked
Skipping v1.38.2 because commit fd294683e7369cabf63738febeaba8a22c925187 has already been checked
Skipping v1.38.3 because commit fd294683e7369cabf63738febeaba8a22c925187 has already been checked
Checking backward compatibility of semgrep_output_v1.atd against past version v1.39.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.40.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.41.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.42.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.43.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.44.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.45.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.46.0
Checking backward compatibility of semgrep_output_v1.atd against past version v1.47.0
Skipping v1.48.0 because commit 278ed753e0c66b8bfc3f2d805fde53be022dd4b6 has already been checked