add ability to customize special character escaping

[vishna]

What is this PR about?

Currently all special characters (that are part of markdown syntax) are getting escaped, e.g. . becomes \. when converting from quill to markdown. This is a reasonable but not always a desirable approach.

This PR adds customization of this behavior.

Description of changes

• Move the escaping logic from visitText to a separate static method called escapeSpecialCharacters
• Create signature for the method called CustomContentHandler and make it available as a constructor parameter of DeltaToMarkdown
• Make the constructor use escapeSpecialCharacters by default
• Add secondary static method called escapeSpecialCharactersRelaxed that conforms to the signature of CustomContentHandler and also escapes special characters, but only when text is styled, meaning fullstops should be omitted.
• Add unit test to illustrate the issue and validate the implementation.

[vishna]

@TarekkMA done

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 94.20%. Comparing base (f8889f9) to head (a34470c). Report is 3 commits behind head on master.

Files	Patch %	Lines
lib/src/delta_to_markdown.dart	95.45%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #31 +/- ## ========================================== - Coverage 94.27% 94.20% -0.07% ========================================== Files 5 5 Lines 454 466 +12 ========================================== + Hits 428 439 +11 - Misses 26 27 +1 ```

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