Refactor the TypeScript data flow for full type safety and auto-generation of Rust types (#3865)

* Migrate Specta to Tsify to auto-generate messages.ts, working except colors and widgets

* Adopt the generated FillColor/Color/GradientStops

* Fix widget typing

* Separate WidgetGroup enum variants into wrapper structs

* Small rename

* Simplify widgets further

* Clean up message type references

* Switch type imports to the auto-generated file

* Remove lowercase serde rename

* Fix FillChoice deserialization

* Fix small regression from #3837

* Improve type safety

* Make WidgetSpan type-safe

* More cleanup and type safety

* More type safety

* More type safety

* Get the rest to type-check without errors; improve widget builder macro to have optional icons; improve Svelte 5 configs

* Cargo fmt

* Fix imports

* Update outdated readme info

* Fix lint command rename references

* Fix typos

* One more typos fix

* Remove unnecessary dep: prefix from the edited Cargo.toml files

* Remove excess parts from Cargo.toml

* Fix compiling on desktop

* Revert "Remove excess parts from Cargo.toml"

This reverts commit 6b711117b3a5d5d8a3ee20f36a43bc74930b7c82.

* Update dev docs with simpler, more accurate instructions

editor/src/dispatcher.rs

@@ -596,7 +596,7 @@ mod test {
 				} = response
 				{
 					if let DiffUpdate::Layout(sub_layout) = &diff[0].new_value {
-						if let LayoutGroup::Row { widgets } = &sub_layout.0[0] {
+						if let LayoutGroup::Row(WidgetRow { widgets }) = &sub_layout.0[0] {
 							if let Widget::TextLabel(TextLabel { value, .. }) = &*widgets[0].widget {
 								print_problem_to_terminal_on_failure(value);
 							}