[TASK] Introduce string fragment extraction

When working with variable interpolation and similar scenarios, in
most cases variables, constants, expressions, ... are embedded in
a solid string and can only be identified and extracted by the
corresponding "reader" or "parser".

This string fragment splitter aims to introduce a simpler way for
extracting and working with these embedded fragments.

Example:
    $pattern = new StringFragmentPattern(
      StringFragmentSplitter::TYPE_EXPRESSION,
      '%[^%]+%'
    );
    $splitter = new StringFragmentSplitter($pattern);
    $collection = $splitter->split(
      'Hello %variable% World!'
      FLAG_UNMATCHED_AS_NULL
    );

    // results in having
    // + StringFragment(type: 'raw', value: 'Hello ')
    // + StringFragment(type: 'expression', value: '%variable%')
    // + StringFragment(type: 'raw', value: ' World!')

Resolves: #97553
Releases: main, 11.5, 10.4
Change-Id: Ie2b02a247ca884fa44ab7b3ba21214c8ee9bc457
Reviewed-on: https://review.typo3.org/c/Packa...