fix(parser): consolidate source-span parser

[PSU3D0]

Summary

• remove the legacy token-vector parser and make the source-span parser the public Parser
• fix leading empty function arguments like =FOO(,A1:C3) and preserve empty-argument arity
• add public parser idioms: Parser::new, builder, TokenStream parsing, FromStr, and TryFrom for ASTNode/Parser
• update bindings/cffi/eval call sites and parser docs

Fixes #103.
Closes #77.

Tests

• cargo fmt --all
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p formualizer-parse
• cargo test --workspace --exclude formualizer-python

Note: cargo test --workspace reaches link but fails for formualizer-python in this environment because -lpython3.13 is unavailable.