Module parser/preprocess/whitespace

Whitespace normalization preprocessing for Wikidot markup.

This module ensures the lexer and parser receive input with consistent whitespace conventions. It handles platform differences (DOS/Mac newlines), normalizes exotic whitespace characters that users may paste from external sources, and applies Wikidot-specific behaviors like backslash line continuation.

Substitutions are applied in a deliberate order:

1. Newline normalization (DOS \r\n and legacy Mac \r to Unix \n)
2. Non-standard leading whitespace replacement (nbsp, figure space to regular space)
3. Whitespace-only line stripping (collapse to empty lines)
4. Backslash line continuation (\\\n to line-break marker U+E000)
5. Tab expansion (tab to four spaces)
6. Null character replacement (NUL to space)
7. Leading/trailing newline removal