add tidb_enable_window_function can control the keyword list

[vzong]

variable tidb_enable_window_function可以控制R-Window中的关键词是否在语法树中生效，如果tidb_enable_window_function设置为OFF，则R-Window中的单词将不再作为关键词。

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add tidb_enable_window_function can control the keyword list.

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[ti-chi-bot]

Hi @vzong. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[qiancai]

Hi @vzong, thanks for your contribution. It seems that this PR is duplicated with #19940. Would you please close one of them?

Btw, to get the PR accepted, would you please click https://cla-assistant.io/pingcap/docs-cn?pullRequest=19941 to sign the Contributor License Agreement?

[vzong]

Hi @vzong, thanks for your contribution. It seems that this PR is duplicated with #19940. Would you please close one of them?

Btw, to get the PR accepted, would you please click https://cla-assistant.io/pingcap/docs-cn?pullRequest=19941 to sign the Contributor License Agreement?

Thank you for your guidance.