Re: More new SQL/JSON item methods

Attached are two small fixup patches for your patch set.

In the first one, I simplified the grammar for the .decimal() method.
It seemed a bit overkill to build a whole list structure when all we
need are 0, 1, or 2 arguments.

Per SQL standard, the precision and scale arguments are unsigned
integers, so unary plus and minus signs are not supported. So my patch
removes that support, but I didn't adjust the regression tests for that.

Also note that in your 0002 patch, the datetime precision is similarly
unsigned, so that's consistent.

By the way, in your 0002 patch, don't see the need for the separate
datetime_method grammar rule. You can fold that into accessor_op.

Overall, I think it would be better if you combined all three of these
patches into one. Right now, you have arranged these as incremental
features, and as a result of that, the additions to the JsonPathItemType
enum and the grammar keywords etc. are ordered in the way you worked on
these features, I guess. It would be good to maintain a bit of sanity
to put all of this together and order all the enums and everything else
for example in the order they are in the sql_features.txt file (which is
alphabetical, I suppose). At this point I suspect we'll end up
committing this whole feature set together anyway, so we might as well
organize it that way.