fix(parse): keep the written order of index columns and expressions - #1009

Summary

• In an index that mixes columns and functional expressions, the interpreter pushed every expression first and then every column, so (country, `lower(name)`) became ((lower(name)), "country") in the exported DDL.
• The columns and expressions of each index argument are now sorted back into source order (by token offset) before they are added.
• Added an interpreter test with (a, `lower(b)`, c, `upper(c)`); it fails before the change.

Not touched here: the reversed order of juxtaposed fields like (a)(b) mentioned in the comments, since whether that form should be accepted is still open in #1008.

Issue

Fixes #1005

Lasting Changes (Technical)

• None, only the order of Index.columns changes.

Checklist

• Documentation (if necessary)
• Updated dbml-homepage/static/llms.txt (if docs/features changed)
• Lint Checks Passed
• Unit Tests Passed (npx vitest run in packages/dbml-parse)
• Coverage Tests Passed
• Integration Tests Passed
• Code Review