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

[paulrutter]

Add the following functions to the parser:

• NOW
• GETDATE

Also add CURRENT_TIMESTAMP() to the parser (with parenthesis).

• Adjust failing test as now is a reserved keyword. It was already, it was just not validated.
• Add test

See #1979

Thank you for the time you are putting into AlaSQL!

[paulrutter]

@mathiasrw whenever you have time, could you please check out this PR? Thanks.

[mathiasrw]

Hi Paul. I really like what you did here. Thank you so much. This is probably how we should have done it from the beginning.

The changes in test/test845.js indicate that this is a breaking change so I will bundle this into a bucket of changes waiting for us to bump major version. Probably during the next few months as we are in the work of restructuring the whole source into ES6 friendly components.

[paulrutter]

@mathiasrw Thanks, good to know that will take a while for this to get into a release.
Since we're making changes on this front, it might be worthwhile to also add all other date functions in the same matter (see #1937 (comment)).

Do you agree? If so, i could work on a PR (WiP: https://github.com/AlaSQL/alasql/compare/develop...blueconic:alasql:feature/add-all-date-functions-to-parser?expand=1)

[mathiasrw]

@paulrutter We are talking about all fields named DATE/NOW/GETDATE/YEAR/MONTH having to be wrapped in escape chars, right?

[paulrutter]

@mathiasrw yes, indeed. In backticks/quotes.

[paulrutter]

@mathiasrw See #1982 for additional work on the parser.
One can still use YEAR, MONTH etc without escape chars in DATEADD for example, although one cannot use it as column alias without escape characters.