Accept identifiers starting with underscore

I think we may also want to consider to accept identifiers starting with _

mysql, psql accepts: 

create table _t(_a integer)

meanwhile hive and oracle does not.

the sql2011 standard suggest that the first char can be:

1) An  is any character in the Unicode General Category classes “Lu”, “Ll”, “Lt”, “Lm”, “Lo”, or “Nl”.

which includes the a _ named "low macron"
https://www.fileformat.info/info/unicode/char/02cd/index.htm
but... the "low line"
https://www.fileformat.info/info/unicode/char/005f/index.htm
is in the Pc class...