Add "continuous" mode to the trace op

[jeffc]

This PR adds a "continuous" mode to the trace op, which interpolates the z step down over the perimeter of the path rather than tracing the whole path then plunging straight down:

I wrote it to only have an effect on closed paths, since open paths would still end up with a plunge if we applied the same logic (unless we "zig-zag" back and forth along the open path, which means alternating climb and conventional milling). If you think this should work for open paths too, let me know and I'll update the PR.

[stewartoallen]

using ease down doesn't provide a similar outcome?

[jeffc]

ease down does something similar, but not quite the same: it gets rid of the straight plunge, but still does a "down, then around" motion (even if the "down" is partly smeared along the path):

This PR moves smoothly and continuously around the path without any breaks, which should lead to a more consistent surface finish

[jeffc]

Converting to a draft; this doesn't respect tabs like I thought (it did at one point, but I must have broken it)