[Lens] Constant and dynamic threshold lines

[flash1293]

This PR adds constant and dynamic threshold lines.

It's a very rough POC investigating which kinds of issues pop up.

Configuration:

Static thresholds:

Dynamic thresholds:

Takeaways

• Dynamic threshold are much easier to implement because they are super close to how regular layers work
• Constant thresholds require much more changes, because a lot of code assumes the presence of a datasource layer - still possible to pull off because it's mostly skipping things.
• It would be pretty hard to transition between the two types of layers - is it OK to have separate ones? Will user understand?

[skip ci]

[flash1293]

Moved to #95612