Add horizontal and vertical padding methods.

[hydra]

e.g.

let status_bar = container(text("status bar area"))
    .padding(padding::horizontal(3));

the point of this PR is:

• to improve the API for everyone.
• to make is so that the number, 3 in the example above, is only specified once.
• so that default values do not have to be specified or overridden.

currently the alternatives are as follows:

1 - specification of vertical and horizontal

let status_bar = container(text("status bar area"))
    .padding([0, 3]);

• the reader does know the order and has to refer to the API docs,
• the intention is not clear, is the intention to override the default top/bottom to 0 and set the left/right to 3, or is the intention just to add horizontal padding?

2 - use the struct

let status_bar = container(text("status bar area"))
    .padding(Padding { left: 3, right: 3, ..padding::default() });

• way to verbose
• not readable
• used the word 'padding' 3 times on the same line.
• duplicates the 3 value.

there may be more alternatives, but hopefully the above goes some way to understanding why i feel horizontal and vertical methods are useful.