switch to named, number-based types for units in all text/labeling related interfaces

similar to `tuples.ts` types such as `GLclampf` (which mostly resolve to `number` or array of `number`) units could be encoded in all function signatures, making it easier to understand the code just by signature (even without comments). For now this seems most relevant for text features of webgl-operate. Especially labeling position allows for a multitude of units depending on the label type.

Start with tuples.ts and its GLclampf-style types, then inspect the text features and labeling interfaces in webgl-operate. Identify the unit variations used by each signature and define a consistent named, number-based type scheme. Done means the relevant text and labeling function signatures encode their units without relying on comments.

Written by the indexing model from the issue text.