Support more `DateTimeField` variants

[iffyio]

• Adds DATETIME variant to DatetimeField
• Adds support for snowflake abbreviations via Custom variant. https://docs.snowflake.com/en/sql-reference/functions-date-time#supported-date-and-time-parts
• Adds support for BigQuery weekday WEEK(MONDAY) syntax https://cloud.google.com/bigquery/docs/reference/standard-sql/date_functions#extract

[coveralls]

Pull Request Test Coverage Report for Build 8407785234

Details

• 77 of 86 (89.53%) changed or added relevant lines in 6 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 87.882%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/parser/mod.rs	12	13	92.31%
tests/sqlparser_bigquery.rs	9	10	90.0%
src/ast/value.rs	39	46	84.78%
<!--	Total:	77	86

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	3	97.14%
<!--	Total:	3

Totals
Change from base Build 8266982631:	0.01%
Covered Lines:	20524
Relevant Lines:	23354

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💛 - Coveralls