Add collection transformation and sequence value expressions

[SanjanaBoggaramJ]

Summary

• Adds ARRAY/FIRST/OBJECT ... FOR ... END as general expressions, reusing the existing range_expr FOR-binding grammar. Previously ARRAY only worked inside index definitions.
• full_array_expr (used by index_key) now delegates to the new array_transform_expr instead of duplicating it.
• Adds NEXT VALUE FOR / NEXTVAL FOR / PREVIOUS VALUE FOR / PREV VALUE FOR / PREVVAL FOR sequence value expressions.
• Combined in one PR since both add an alternative to the same expr rule.

Solves the Sequence value and collection transformation issue mentioned in #6

For more info here are the respective doc links:

• https://docs.couchbase.com/server/current/n1ql/n1ql-language-reference/sequenceops.html
• https://docs.couchbase.com/server/current/n1ql/n1ql-language-reference/collectionops.html

Test plan

• Added tests for ARRAY/FIRST/OBJECT transforms, the CREATE INDEX regression path, and all 5 sequence-expr synonyms.
• Confirmed CollectionExtractor doesn't misinterpret the new var/name_var bindings as keyspace paths.
• Full existing test suite passes with no regressions.
• Found two pre-existing, unrelated RETURNING bugs — filed separately.

[Copilot]

nope.