Add math.round; fix vector.round

[v-rob]

Adds math.round to round to the nearest integer (simple, but useful), rounding away from zero at multiples of 0.5, as well as making vector.round follow this behaviour instead of rounding multiples of 0.5 towards +inf.

Fixes #6774

[Desour]

• Imo, it should be documented whether x.5 is rounded towards 0, +inf, -inf or +-inf. Not only for these functions, but also eg. for get_node.
• I wouldn't call this a "fix" of vector.round. Some mods might rely on the current behaviour.
• Imho rounding x.5 towards +-inf is not objectively better than towards +inf. But math.floor(x+0.5) is a little faster.
• Your implementation of math.round rounds -0.5 to math.ceil(-0.5-0.5)=-1, which is "incorrect" according to you.

[v-rob]

• Imho rounding x.5 towards +-inf is not objectively better than towards +inf. But math.floor(x+0.5) is a little faster.

Technically, there are multiple ways of rounding a number. However, in practice, I have only ever learned or heard anyone use one rule: to round 4.569 to an integer, look at the first decimal point. If it is <=5, round up. If it is >5, round down. That rule makes no exception for negative numbers. If I'm incorrect and some people use other rules in daily life, please correct me.

Different programming languages handle this differently. C and C++ round away from zero (like this PR), Python rounds towards zero (the opposite of this PR), JavaScript rounds towards positive infinity (like vector.round), etc.

I don't care particularly much; I just chose this implementation because I thought it was the one most people were familiar with. I just thought it would be nice to have a rounding function built in. (I mean, we have vector.round but not math.round? That's kind of ridiculous). I'd rather just get some sane rounding function in without arguing about the most superior rounding rule.

• Your implementation of math.round rounds -0.5 to math.ceil(-0.5-0.5)=-1, which is "incorrect" according to you.

That's a typo on the PR description; I meant to say that vector.round rounds -0.5 to 0. Fixed.

[ghost]

I use round a lot. Necessary IMO.

[rubenwardy]

Following C++'s example makes sense, especially if you want to round node positions

For reference:

Input	-0.5	-1.5	-2.5	-3.5	Note
C++	-1	-2	-3	-4	Rounds from 0
Python	0	-2	-2	-4	WTF?
JavaScript	0	-1	-2	-3	Rounds to inf
Lua (+0.5)	0	-1	-2	-3	Rounds to inf
Lua (PR)	-1	-2	-3	-4	Rounds from 0

I suggest not following Python's example

[v-rob]

Added docs saying it rounds away from zero at 0.5.

[SmallJoker]

LGTM