ts api

[Fil]

rebooting #1001 with a "typescript as documentation" approach.

Things to watch for:

• [ ] Plot.valueof(undefined, () => {}) crashed
• [ ] Plot.map("foo") crashed
• [x] Plot.selectMinX potentially inserted undefined in the facet index
• [x] mid(x1, x2) was wrongly typed (it expected a data argument)
• [x] the API for marker passed a Context as {document}, now just passes document: see e891948
• [x] Plot.stackY crashes on some orders: ("z", "appearance", etc) and X or Y are missing
• [x] The window transform used the percentile function with the wrong types

[Fil]

This PR would be monstruous to review. I'm leaving it as a draft to show the context, but I'll submit it separately in digestible pieces. The first piece will be data.ts + options.ts

[mbostock]

Probably superseded by #1320 which goes the d.ts route. Close?