Export a design's timeline as a replayable command script (#1045)

timeline.export_script (and solidcraft-cli script) writes a design's history as a command script that solidcraft-cli eval replays into the same design, so a bug that only shows on one design becomes a one-file repro. timeline.add restores one stored feature with its id, names and references, through the same checks a design file gets when it opens.

Closes #886. SychPL's #1078 approached the same issue independently; thanks to both.

Co-authored-by: SychPL <SychPL@users.noreply.github.com>

crates/doc/src/params.rs

@@ -459,7 +459,7 @@ impl Document {
         v
     }
 
-    fn name_taken(&self, n: &str) -> bool {
+    pub(crate) fn name_taken(&self, n: &str) -> bool {
         self.param(n).is_some() || self.features.iter().any(|f| f.param_names.iter().any(|x| x == n))
     }