flexmark-java extension for flexmark-java spec test example processing

Converts spec example syntax into flexmark-java AST node.

Syntax:

```````````````````````````````` example SpecExample: 1
markdown content
.
html content
.
AST content
````````````````````````````````

Example test spec:

```````````````````````````````` example GFM Emphasis: 7
**a*b*c**

.
<p><strong>a<em>b</em>c</strong></p>
.
Document[0, 11]
  Paragraph[0, 10] isTrailingBlankLine
    StrongEmphasis[0, 9] textOpen:[0, 2, "**"] text:[2, 7, "a*b*c"] textClose:[7, 9, "**"]
      Text[2, 3] chars:[2, 3, "a"]
      Emphasis[3, 6] textOpen:[3, 4, "*"] text:[4, 5, "b"] textClose:[5, 6, "*"]
        Text[4, 5] chars:[4, 5, "b"]
      Text[6, 7] chars:[6, 7, "c"]
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