Add magic __set method to Markdown pages to set data in front matter

[caendesilva]

Companion to #339

[codecov]

Codecov Report

Merging #343 (92b9abd) into master (76dd0c7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #343   +/-   ##
=========================================
  Coverage     99.85%   99.85%           
- Complexity      832      834    +2     
=========================================
  Files            95       95           
  Lines          2052     2057    +5     
=========================================
+ Hits           2049     2054    +5     
  Misses            3        3           

Impacted Files	Coverage Δ
...s/framework/src/Contracts/AbstractMarkdownPage.php	100.00% <100.00%> (ø)
packages/framework/src/Models/FrontMatter.php	100.00% <100.00%> (ø)

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