feat(ie-css): implement CSS tokenizer per CSS Syntax Level 3 #67

CSS tokenizer with all token types per the spec:
- CssToken enum: Ident, Function, AtKeyword, Hash(Id/Unrestricted),
  String, BadString, Url, BadUrl, Delim, Number(Integer/Number),
  Percentage, Dimension, Whitespace, Cdo, Cdc, punctuation, Eof
- Input preprocessing: CR/LF normalization, null replacement
- Escape handling: hex escapes (up to 6 digits), literal escapes
- Number consumption: integer, float, scientific notation
- String tokens: single/double quoted with escape handling
- URL tokens: unquoted URL consumption
- Ident-like: identifier, function, url() detection
- Comments consumed and discarded
- Iterator-based: impl Iterator<Item = CssToken>
- 26 unit tests covering all token types and edge cases

Author: thomasnemer

crates/ie-css/src/lib.rs

@@ -6,6 +6,8 @@
 pub mod parser;
 pub mod selector;
 pub mod style;
+pub mod tokenizer;
 
 pub use parser::parse_stylesheet;
 pub use style::ComputedStyle;
+pub use tokenizer::{CssToken, CssTokenizer};