End a quoted paragraph in a list item at a bare quote marker

[dereuromark]

Inside a list item or a definition, a quote line holding only > did not close the quoted paragraph, so the next unindented line was taken as lazy text and stayed in the item:

- > a
  >
b

djot.js ends the list and makes b a paragraph after it. djot-php kept b in the item. The same happened in a definition (: t, blank, > a, >, b) and with nested quotes.

The syntax description: the contents of a block quote "(minus initial >) are parsed as block-level content", so a bare > is a blank line inside the quote, and a "paragraph ends with a blank line or the end of the document". Once the paragraph is closed, b cannot continue it lazily. The item tracker now remembers how many quote levels the open paragraph sits in and closes it on a line with nothing after those markers. A > after plain item text is still text, as before.

Documents with this shape render differently, so this is labeled bc-breaking.

Measured against djot.js

On the 4853 generated block-family inputs, djot-php differed from djot.js 9790d4d on 852 after #316 and 849 with this change. None moved the other way. On the 1080 list item bodies from #316 nothing changed.

The new tests keep <p> inside a quote in a tight list item, where djot.js renders the text bare. That is the djot.js behavior that gives nested blocks the outer list's tightness, which this PR does not follow.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.13%. Comparing base (5b9c33c) to head (64956db).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Parser/BlockParser.php	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #317   +/-   ##
=========================================
  Coverage     93.13%   93.13%           
- Complexity     4757     4770   +13     
=========================================
  Files           118      118           
  Lines         12706    12721   +15     
=========================================
+ Hits          11834    11848   +14     
- Misses          872      873    +1     

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