Add native grammar registration and external crate guide

[fasterthanlime]

Summary

This adds a minimal public API for registering third-party compiled grammars into Arborium's shared GrammarStore.
It also documents the outside-the-repo workflow for packaging an Arborium-compatible grammar crate and includes a full standalone example crate that registers itself into Arborium.

Changes

• add GrammarStore::insert(...) to the generated store template with explicit normalization and override semantics
• document that Highlighter::with_store(...) and related shared-store flows pick up registered grammars
• add focused tests for custom grammar registration, coexistence with built-ins, and duplicate insert behavior
• add EXTERNAL_GRAMMAR_CRATES.md explaining how to build an Arborium-compatible grammar crate from outside this repo
• add examples/arborium-example-json as a full standalone example crate with a runnable Arborium integration example
• link the new external-crate guide from the Arborium docs

Testing

• cargo nextest run --manifest-path /Users/amos/bearcove/arborium/crates/arborium/Cargo.toml --features lang-rust
• cargo check --manifest-path /Users/amos/bearcove/arborium/examples/arborium-example-json/Cargo.toml --example highlight_with_arborium
• cargo run --manifest-path /Users/amos/bearcove/arborium/examples/arborium-example-json/Cargo.toml --example highlight_with_arborium