feat: Allow EXTRACT field to be parsed as a string

[MazterQyou]

This PR introduces a change that allows EXTRACT expression to accept date/time fields as strings, e.g. EXTRACT('year' FROM column). PostgreSQL allows those kinds of strings.
Although it also adds a related test, the test suite is half-baked; see the comment in parse_pg_extract function.

[coveralls]

Pull Request Test Coverage Report for Build 3265165931

• 40 of 44 (90.91%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.009%) to 85.183%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_postgres.rs	6	7	85.71%
src/parser.rs	34	37	91.89%

Totals
Change from base Build 3090366902:	-0.009%
Covered Lines:	8779
Relevant Lines:	10306

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💛 - Coveralls

[ovr]

Why you cannot move this check into parse_date_time_field method?)

[MazterQyou]

parse_date_time_field is also used with intervals, for instance, where such string behavior would be explicitly not allowed in Postgres. INTERVAL '1' MONTH is ok, but INTERVAL '1' 'MONTH' would be an error. If I were to move the check, such incorrect syntax would be parsed as valid.

[ovr]

LGTM