refactor: replace stat system with variable system

Replaces the specialized `stat` system with a more general-purpose
variable system. This includes:

- Renaming `ReactiveStatManager` to `ReactiveVariableManager`
- Changing template syntax from `${key}` to `{{$key}}`
- Replacing `StatsView` with `VariableView` to show declared, plain,
  and tag-typed variables
- Updating command `/stat` to `/set`
- Refactoring the reactivity engine to support variable declarations
  and tag modifiers via `csv role=declare` blocks

src/components/CommandLinkManager.tsx

@@ -32,9 +32,9 @@ export const CommandLinkManager: Component = () => {
       myName: stream.myName,
       command,
       sparkTables: comp.data.sparkTables,
-      statValues: stream.stats,
-      statDefs: comp.data.stats,
-      statTemplates: comp.data.statTemplates,
+      variables: stream.variables,
+      declarations: comp.data.declarations,
+      tagModifiers: comp.data.tagModifiers,
     }).then((result) => {
       if (!result.ok) {
         setDispatchError(result.error);