Add functional utilities for styles that need numbers

[JoshKarpel]

We recently added utilities for positioning that are functions

def relative(x: int = 0, y: int = 0) -> Style:
    return Style(layout=Flex(position=Relative(x=x, y=y)))


def absolute(x: int = 0, y: int = 0) -> Style:
    return Style(layout=Flex(position=Absolute(x=x, y=y)))


def fixed(x: int = 0, y: int = 0) -> Style:
    return Style(layout=Flex(position=Fixed(x=x, y=y)))

We also have older utilities like

gap_children_0 = Style(layout=Flex(gap_children=0))
gap_children_1 = Style(layout=Flex(gap_children=1))
gap_children_2 = Style(layout=Flex(gap_children=2))
gap_children_3 = Style(layout=Flex(gap_children=3))
gap_children_4 = Style(layout=Flex(gap_children=4))
gap_children_5 = Style(layout=Flex(gap_children=5))
gap_children_6 = Style(layout=Flex(gap_children=6))
gap_children_7 = Style(layout=Flex(gap_children=7))
gap_children_8 = Style(layout=Flex(gap_children=8))

These are convenient for small, common numbers, but we should also have

def gap_children(n: int) -> Style:
    return Style(layout=Flex(gap_children=n))

There are a variety of utilities that end with a trailing number, and each should probably have a functional variant as well.

Also, all functional utilities should be cached, so that we re-use Style objects as much as possible (helps style merge caching).