Fix bad indentation detection for block collection entries

[fktn-k]

This PR fixes detecting bad indentations for block sequence/mapping entries as reported in the issue #449.
Also, the way of managing parse contexts has been improved by separating BLOCK_SEQUENCE_ENTRY from BLOCK_SEQUENCE in the enum class context_state_t for the above fix.
That allows for block sequence entries to be empty and they are now properly parsed.

# The following YAML was parsed as `[foo, [123], bar]` before.
# Now the result is `[foo, [123, null], null, bar]`.
- foo
- - 123
  -
-
- bar

---

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• The test suite compiles and runs without any error.
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