feat: add clean_newline to concatenate words wrapped across lines

[TrueFurina]

Summary

Per #2513: when text is wrapped, a word that continues on the following line is often written as a character followed by a dash and whitespace. Adds a clean_newline function that removes the hyphen and whitespace and joins the two parts.

Changes

• unstructured/cleaners/core.py: new clean_newline(text, pattern=r"(\w+)-\s+(\w+)") using re.sub(pattern, r"��", text), exactly as proposed in the issue
• test_unstructured/cleaners/test_core.py: parametrized tests covering hyphen+newline, hyphen+space+newline, plain text passthrough

Verification

• All 5 parametrized cases pass locally (docu-
  ment → document, docu-
  ment → document, hello-
  world → helloworld, no-hyphen text unchanged, word-
  word → wordword)
• ast.parse passes on both files
• Implementation matches the issue's proposed signature/docstring intent

[mi7ak2020-stack]

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