FrontmatterExtension: Support block attributes and spaced syntax

[dereuromark]

Summary

• Adds consumePendingAttributes() to BlockParser for extensions to access pending block attributes
• FrontmatterExtension now supports standard djot block attribute syntax on the line above
• Removes inline attribute syntax (---yaml {.meta}) in favor of standard djot block attributes

New Syntax

Block attributes are placed on the preceding line (standard djot style):

{.meta #frontmatter}
---yaml
title: My Document
---

{kernel="myproject" #cell-1}
---python
import flight
---

Follow-up to #122, based on discussion in jgm/djot#35.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.92%. Comparing base (36dbbcb) to head (e27b71c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #124   +/-   ##
=========================================
  Coverage     93.92%   93.92%           
- Complexity     2382     2383    +1     
=========================================
  Files            81       81           
  Lines          6303     6306    +3     
=========================================
+ Hits           5920     5923    +3     
  Misses          383      383           

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