add default value for window frame

[Jimexist]

@andygrove please take a look, thanks!

[coveralls]

Pull Request Test Coverage Report for Build 901793053

• 5 of 6 (83.33%) changed or added relevant lines in 2 files are covered.
• 21 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.1%) to 90.178%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_common.rs	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/ast/data_type.rs	1	75.0%
src/ast/ddl.rs	2	80.18%
src/ast/query.rs	2	86.6%
src/parser.rs	4	85.89%
src/tokenizer.rs	4	91.18%
src/ast/mod.rs	8	70.83%

Totals
Change from base Build 676410191:	0.1%
Covered Lines:	5472
Relevant Lines:	6068

---

💛 - Coveralls

[alamb]

I have rebased this PR to get the CI checks running, and if they pass I will merge it in

[coveralls]

Pull Request Test Coverage Report for Build 1151595220

• 5 of 6 (83.33%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 90.079%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tests/sqlparser_common.rs	0	1	0.0%

Totals
Change from base Build 1151591467:	0.008%
Covered Lines:	5557
Relevant Lines:	6169

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💛 - Coveralls