Fix parsing of conformance test documents with multiple extension clauses (like Then)

[nirosys]

Issue #, if available: #916

Description of changes:
Prior to this PR the conformance DSL would only parse the first Then or Each clause in a document or document-like. This lead to false positives where only the first branch in a document would be evaluated.

This PR adjusts the parsing of documents to correctly pack multiple extension into Continuation::Extensions which handles evaluating the extensions in sequence.

Additionally, this PR adds 3 more skip list entries, and reduces the tests skipped in invoke_ee.ion and invoke_cardinality_ee.ion. The additional skip entries are due to this PR fixing Then sequences, which added previously ignored test branches to the tests.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.72%. Comparing base (51d5be2) to head (e1c55b4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #930      +/-   ##
==========================================
- Coverage   78.76%   78.72%   -0.04%     
==========================================
  Files         137      137              
  Lines       34605    34605              
  Branches    34605    34605              
==========================================
- Hits        27256    27244      -12     
- Misses       5348     5363      +15     
+ Partials     2001     1998       -3     

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