Replace schema traits with interface constants

[caendesilva]

The reason the schema traits were introduced was to clearly separate and define the recognized front matter options, however having it in a trait doesn't really make too much sense as they are not intended for direct reuse, but instead as a a static reference, and since they shouldn't change, why not have them as constants, and why not have those constants in an interface? It's something I only recently realized works, so let's try it out.

The schema traits were an experiment to begin with, and removing them will reduce a lot of complexity and extra code that is not needed. I'm also removing the inline documentation, as the schemas should be stable and not change a lot, and if a property needs to be explained to be understandable, it might need renaming anyways.

A large problem with using the traits for schemas is that they define the internal data type, which may not correspond with the front matter version. For example, an author is always internally parsed into an Author model, but to define one you either need to use a string or array in the front matter.

[codecov]

Codecov Report

Merging #485 (f00be08) into master (84d140c) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #485      +/-   ##
============================================
- Coverage     99.95%   99.95%   -0.01%     
+ Complexity      934      927       -7     
============================================
  Files           108      107       -1     
  Lines          2299     2280      -19     
============================================
- Hits           2298     2279      -19     
  Misses            1        1              

Impacted Files	Coverage Δ
packages/framework/src/Concerns/AbstractPage.php	100.00% <ø> (ø)
...s/framework/src/Models/Pages/DocumentationPage.php	100.00% <ø> (ø)
...ckages/framework/src/Models/Pages/MarkdownPost.php	100.00% <ø> (ø)
...s/framework/src/Concerns/ConstructsPageSchemas.php	100.00% <100.00%> (ø)

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