Support arc tracks (GenericJSON)

[Funkenjaeger]

Support for arc tracks with GenericJSON parser, since Eagle/Fusion 360 Electronics are capable of curved traces.

[qu1ck]

Looks good, some formating nits.

[qu1ck]

Clicking to highlight will break. Need to handle arcs there too.
in render.js netHitScan()

[Funkenjaeger]

Good point, I had sort of forgotten about the click-to-highlight capability. That was a fun little Sunday morning math quiz, but it seems to be working now.

For reference: Eagle/Fusion arc angles (internally) are in the range [0.0, 719.9], following the usual mathematical convention where angle is measured counterclockwise from the positive X axis. Since the Canvas API evidently follows the opposite angle convention, in my Eagle script I simply negated both angles so in the JSON they're always in the range [-719.9, 0.0]. Those values render the arcs in the canvas just fine as-is, but I had to do modulo arithmetic within pointWithinDistanceToArc() when checking proximity. I think all that logic should also work for values outside that numerical range (assuming Kicad will likely be different) but I haven't personally tested that.

(side note: a comment in DATAFORMAT.md indicating the angle convention wouldn't hurt)

[qu1ck]

Thanks for your contribution. I squashed your commits with a few fixes I made while testing with kicad.
Updated DATAFORMAT too.