Change input string handler termination sequence to <<<

[caendesilva]

Changes InputStreamHandler termination sequence from an empty line ('') to '<<<'.

While this is slightly more verbose, it prevents confusion and accidental premature termination as input strings might have empty lines. And the use case for entering paragraphs through the command line is (I assume) so slim that the extra characters are acceptable.

Fixes hydephp/publications#2 and resolves code review comment #685 (comment)

[codecov]

Codecov Report

Merging #936 (cdf3b37) into publications-feature (a286237) will not change coverage.
The diff coverage is n/a.

@@                    Coverage Diff                     @@
##             publications-feature      #936     +/-   ##
==========================================================
  Coverage                  100.00%   100.00%             
+ Complexity                   3192      1597   -1595     
==========================================================
  Files                         348       174    -174     
  Lines                        7958      3981   -3977     
==========================================================
- Hits                         7958      3981   -3977     

Impacted Files	Coverage Δ
...ations/src/Commands/Helpers/InputStreamHandler.php	100.00% <0.00%> (ø)
...ations/src/Commands/Helpers/InputStreamHandler.php
...blications/src/Commands/MakePublicationCommand.php
...work/src/Framework/Concerns/ValidatesExistence.php
packages/framework/src/Pages/DocumentationPage.php
...rk/src/Framework/Factories/BlogPostDataFactory.php
.../src/Framework/Factories/NavigationDataFactory.php
...src/Framework/Exceptions/FileNotFoundException.php
...k/src/Framework/Views/Components/LinkComponent.php
.../Framework/Features/Blogging/Models/PostAuthor.php
... and 165 more

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