Add dbt templating support

[jhklarcher]

Summary

Adds explicit dbt templating support as a mode layered on the existing SQL dialects. The default Jinja behavior remains passthrough, while --templating dbt protects dbt/Jinja tags, formats surrounding SQL, and restores tags exactly.

Changes

• Adds templating = "passthrough" | "dbt" config and --templating <passthrough|dbt> CLI support.
• Adds dbt-aware Jinja preservation for expressions, blocks, comments, trim markers, raw blocks, and semicolons inside tags.
• Adds dbt default directory includes for .sql, .sql.j2, .sql.jinja, and .sql.jinja2.
• Adds dbt ANSI and Dremio fixtures plus reference syntax fixtures.
• Adds docs/dbt-support-plan.md as the trackable milestone/task plan and links it from docs.

Validation

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test (99 passed)