Sketch text: bold and italic, text along a curve, alignment and spacing (#852)

A sketch text gets an optional style: bold and italic faces (a slanted fallback for italic), left/center/right alignment, letter and line spacing, and a path curve of the same sketch for the baseline to follow (reverse takes the other side). Edit Text gets B/I toggles, alignment, spacing fields and a Path chip, all live. The style is saved only when not plain and is noted under format 18.

crates/doc/src/format.rs

@@ -108,6 +108,11 @@
 //!   bodies) and `ReverseNormal` (a surface body's faces turned over), each a `body` and
 //!   optional `faces` points. Version 17 designs need no transformation; the version stamp makes
 //!   0.9.0 (which reads up to 17) ask for an update instead of meeting a feature it can't read.
+//!   Under the same version, a sketch text's link source can carry a `style`: `bold` and
+//!   `italic` (the font's bold or italic face), `align` (`left`, `center` or `right`),
+//!   `letter_spacing` (mm), `line_spacing` (times the font's line distance), `path` (the id of a
+//!   curve of the same sketch the baseline follows) and `reverse`. A text without `style` is set
+//!   as it always was; 0.9.0 would drop the style and set the text plain, off its curve.
 
 use serde_json::Value;