Add support for INTERVAL keyword as unquoted identifier in PostgreSQL

[LucaCappelletti94]

This PR fixes a PostgreSQL dialect parsing bug where INTERVAL was always treated as a reserved identifier keyword, causing valid queries such as:

SELECT MAX(interval) FROM tbl

to fail with Expected: an expression, found: ).

Problem

In PostgreSQL, interval can be used as an unquoted identifier in unambiguous expression positions (for example function arguments). Our parser attempted INTERVAL-literal parsing first, and on failure it only fell back to identifier parsing when the keyword is not reserved. Because INTERVAL was reserved for PostgreSqlDialect, fallback was blocked.

Fix

• Added a PostgreSQL-specific override of is_reserved_for_identifier so Keyword::INTERVAL is not treated as reserved in this dialect.
• Kept default reserved behavior for all other keywords unchanged.

Tests

Added regression test in PostgreSQL-specific suite:

• SELECT MAX(interval) FROM tbl now parses successfully in PostgreSqlDialect.
• INTERVAL '1 day' still parses as an interval expression (guarding against regressions).

Impact

• Improves compatibility with real-world PostgreSQL schemas where columns/args may be named interval.
• No public API changes.
• No cross-dialect behavior changes intended beyond PostgreSQL’s INTERVAL identifier handling.