Fix lalrpop-util missing the "lexer" feature under Cargo resolver v2

[KoenZandvlietAltius]

Problem

Consuming scallop-core as a plain Rust dependency from a project using Cargo's resolver v2 (the default for any crate on edition 2021+) fails to compile:

error[E0433]: cannot find `lexer` in `__lalrpop_util`

Cause

core/Cargo.toml requests lalrpop's "lexer" feature only under [build-dependencies] (used to generate the parser code from the grammar), but [dependencies]'s lalrpop-util (used by the generated code itself, at normal compile time) doesn't request that same feature:

[build-dependencies]
lalrpop = { version = "0.19.9", features = ["lexer"] }

[dependencies]
lalrpop-util = "0.19.9"  # missing features = ["lexer"]

Under Cargo's older resolver, build- and normal-dependency features were unified, so this went unnoticed. Resolver v2 correctly keeps them separate, so the generated grammar.rs can no longer find __lalrpop_util::lexer::MatcherBuilder. This doesn't surface when building scallop-core inside this repo's own workspace, only when it's pulled in as a dependency by a resolver-v2 consumer.

Fix

[dependencies]
lalrpop-util = { version = "0.19.9", features = ["lexer"] }

Verification

Found while embedding scallop-core as a Rust dependency in another project. Confirmed with cargo tree -i lalrpop-util, which shows the crate resolved twice — once with the feature (via the lalrpop build-dependency), once without (via scallop-core's own direct dependency). Verified both directions against that consuming project: reverting this change reproduces the exact 17 E0433/E0432 errors again, and applying it resolves them cleanly.