Add support for constraint characteristics clause

[dimfeld]

This adds support for [[NOT] DEFERRABLE] [INITIALLY DEFERRED | INITIALLY IMMEDIATE] [[NOT] ENFORCED] on constraint clauses.

Syntax references:

• https://jakewheat.github.io/sql-overview/sql-2016-foundation-grammar.html (10.8)
• https://www.postgresql.org/docs/16/sql-createtable.html#SQL-CREATETABLE-PARMS-DEFERRABLE
• https://www.sqlite.org/foreignkeys.html (4.2)

[coveralls]

Pull Request Test Coverage Report for Build 7633270040

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• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 87.65%

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Totals
Change from base Build 7632579793:	0.06%
Covered Lines:	19397
Relevant Lines:	22130

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💛 - Coveralls

[alamb]

Thank you very much for this contribution. I have just one request to not modify the existing tests (but instead to add a new test).

I plan to make a release later this week, for your planning purposes.

[dimfeld]

Updated. Thanks for the feedback!

[alamb]

Looks good to me -- thanks for the quick turnaround @dimfeld