Add svg-path API to support SVG-like path object

[liuguangxi]

With the introduction of curve in Typst 0.13, it's now possible to construct a path primitive with fully functional.

It would be nice to have a draw function that corresponds to the SVG path.

For example, consider the following SVG path:

<path d="M 10 80 C 40 10, 65 10, 95 80 S 150 150, 180 80 Z" stroke="black" fill="transparent"/>

It could be implemented using svg-path as below:

svg-path(
    stroke: black, fill: none,
    ("M", (10, 80)),
    ("C", (40, 10), (65, 10), (95, 80)),
    ("S", (150, 150), (180, 80)),
    ("Z"),
)

The command in SVG path:

• M/m = moveto
• L/l = lineto
• H/h = horizontal lineto
• V/v = vertical lineto
• C/c = curveto
• S/s = smooth curveto
• Q/q = quadratic Bézier curve
• T/t = smooth quadratic Bézier curveto
• A/a = elliptical arc
• Z/z = closepath

where uppercase letters indicate absolute positioning, and lowercase letters indicate relative positioning.

Except the command A/a (elliptical arc), other command can be mapped directly to a curve element.

The advantage is that it is easy to construct complex path objects (manually or programmatically) concisely.

In addition, it is convenient to realize the automatic conversion of SVG path to CeTZ code (Maybe there will be a tool like SVG2CeTZ?).

[johannes-wolf]

The internal path representation of cetz has to be changed first, but this is a nice feature to have.