More YAML parse fixes

[biojppm]

This PR continues the previous work from #586 and #585.

The changeset is gigantic, but the overwhelming majority of it is adding new tests, as well as moving tests across different test files when they become too large for compilation.

There are no API changes, and the relevant changes are all internal to the parser.

The PR address many existing YAML issues, notably:

• Fix parsing of valid YAML corner cases:
  • Ensure tags/anchors are not omitted. This was happening in some flow maps.
  • Ambiguity of tags/anchors in ? mode:

    ? &mapanchor
      key: val
    ?
      &keyanchor key: val

  • flow tags/anchors with omitted plain scalar:

    # ... likewise for !tag
    - [&anchor,&anchor]
    - {&anchor,&anchor}
    - [&anchor :,&anchor :]
    - {&anchor :,&anchor :}
    - [: &anchor,: &anchor]
    - {: &anchor,: &anchor}
    ---
    ? anchor

---

• Ensure parse errors for invalid YAML cases, and improve reported location:
  • doc start/begin tokens at zero indentation in seq flow:

    [
    ---,
    --- ,
    ---\t,
    ...,
    ... ,
    ...\t,
    # etc
    ]

  • nested flow containers now enforce the contextual parent indentation:

    - - - [
        a  # now this is a parse error
         ]
    - - - [
         a  # this is ok
         ]

  • single/double-quoted scalars now enforce the contextual parent indentation:

    - - - "a
        b"  # now this is a parse error

  • plain scalars in block mode starting with ,:

    all invalid:
      - , foo
      - ,foo
      - ,

  • references with anchors or tags:

    all invalid:
      - &anchor *ref
      - !tag *ref

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.50%. Comparing base (c842cf7) to head (b076d39).
⚠️ Report is 18 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #587      +/-   ##
==========================================
+ Coverage   97.70%   98.50%   +0.79%     
==========================================
  Files          46       46              
  Lines       13790    13948     +158     
==========================================
+ Hits        13474    13739     +265     
+ Misses        316      209     -107     

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