rustc-like errors with file/span context

[dmtrshat]

• Added rich, rustc-like diagnostics: render errors with file context + labeled span + optional help text.
• Introduced a SourceFile container to carry path/name + full content so errors can always attach a readable source snippet.
• Extended parsing to track spans (start, len) for each extracted class/spell token, and propagate that through spell generation:
  • Spans collected in parser_base.rs.
  • FS parser wraps errors with source content in parser_fs.rs.
  • Spans/source carried by Spell in spell.rs.
• Upgraded the error model to support structured compile errors (message/label/help/span/source) instead of only plain strings:
• Added a dedicated diagnostics adapter mapping GrimoireCssError -> miette::Diagnostic.
• Improved common user mistake messaging (especially HTML space-splitting) and earlier validation:
  • Better errors for malformed function-like values / parentheses: spell_value_validator.
  • Color function argument validation now returns a proper error (instead of silently ignoring): color_functions.rs.
• Added an error_scenarios sandbox with minimal repro HTML/config to quickly verify diagnostics quality.

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closes #95