Fix for parsing comments in flow mode

[joshuapare]

Fixes issue #608 where comments in flow-style YAML (using [] and {}) were not being captured by CommentToMap.

This PR fixes the following behavior:

• comments on closing tokens weren't being stored as FootComments on the AST nodes
• decoder wasn't processing entry-level comments in flow-style sequences
• comma-line comments in flow blocks were incorrectly assigned to the next element instead of the element before the comma (e.g., the comment after a, should belong to element at comma[0], not element b at comma[1])

For example:

flow_seq: [a, b, c]  # was captured
flow_map: {x: 1, y: 2}  # was captured
comma: [a, # was NOT captured
        b, # was NOT captured
        c] # was NOT captured
nested: {
  key: [1, 2]  # was NOT captured
}  # was NOT captured
empty: []  # was NOT captured
single: [x]  # was NOT captured
nested_seq: [[a, # was captured but DUPLICATED on element b 
              b], # was captured on WRONG element (outer entry instead of inner FootComment) 
              c] # was NOT captured on element c
mixed: {
  list: [1, 2], # was captured
  map: {x: y} # comment was captured BUT also leaked to $.mixed.map.x
}

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 70.21277% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.40%. Comparing base (b0ab069) to head (eb5a563).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #814      +/-   ##
==========================================
- Coverage   80.48%   80.40%   -0.09%     
==========================================
  Files          22       22              
  Lines        6845     6919      +74     
==========================================
+ Hits         5509     5563      +54     
- Misses        914      930      +16     
- Partials      422      426       +4     

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