Add SemanticSpanExtension for kbd, dfn, abbr elements

[dereuromark]

Summary

• Adds SemanticSpanExtension that converts span attributes into semantic HTML5 elements
• Supports kbd (keyboard input), dfn (definitions), and abbr (abbreviations)
• Adds getChildrenHtml() to RenderEvent for extensions that need to wrap children in different HTML elements

Syntax Examples

Press [Ctrl+C]{kbd} to copy.
A [variable]{dfn} is a named storage location.
The [HTML]{abbr="HyperText Markup Language"} standard.
[CSS]{dfn abbr="Cascading Style Sheets"}

Output:

<kbd>Ctrl+C</kbd>
<dfn>variable</dfn>
<abbr title="HyperText Markup Language">HTML</abbr>
<dfn><abbr title="Cascading Style Sheets">CSS</abbr></dfn>

Inspired by php-collective/wp-djot#5

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.81%. Comparing base (a6e227a) to head (a2e211c).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Event/RenderEvent.php	87.50%	1 Missing ⚠️
src/Extension/SemanticSpanExtension.php	97.29%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #77      +/-   ##
============================================
+ Coverage     93.76%   93.81%   +0.05%     
- Complexity     2077     2095      +18     
============================================
  Files            76       77       +1     
  Lines          5595     5641      +46     
============================================
+ Hits           5246     5292      +46     
  Misses          349      349              

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