Add GETDATE, NOW as alias to parser + allow CURRENT_TIMESTAMP with parenthesis

[paulrutter]

We're in the process of using the AlaSQL parser to validate query syntax in our product.
The AlaSQL parser currently does not recognize the following aliases to CURRENT_TIMESTAMP (see 61date.js).

• NOW
• GETDATE

Also, CURRENT_TIMESTAMP is recognized by the parser, while CURRENT_TIMESTAMP() is not.
Do note that all of these functions do work in a query, the parser just cannot properly validate their use.

I'm going to supply a PR to get this fixed, which has one drawback: now, current_timestamp and getdate can no longer be used as column aliases, as they're now reserved keywords.

Let me know what you think @mathiasrw.

[paulrutter]

Ps. The same applies to other date functions like DAY() and YEAR(), but that would break far more queries (although it would be more consistent). I tried that locally a while ago. See #1937 (comment)

With the proposed PR, at least all functions for getting the current date (+time) are consistently handled.