Avoid building a temporary Rust token vector before calling back into
C++. The tokenizer now invokes the callback as each token is produced,
while borrowing the already-filtered input for source slices.
Reserve an initial C++ token capacity from the input size so the common
path avoids repeated growth while appending the converted tokens.
With this change, the Rust CSS tokenizer is now ~1.3x faster than the
C++ CSS tokenizer at churning through all the https://vercel.com/ CSS.
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