HS: viewTransition command (+9 tests)

Add 'start view transition [using EXPR] [then] BODY end' syntax.
- tokenizer: add 'view' as a keyword
- parser: add 'start' to cmd-kw? and dispatch to view-transition! AST node
- compiler: emit hs-view-transition! call from view-transition! node

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/hyperscript/compiler.sx

@@ -1832,6 +1832,9 @@
                     (hs-to-sx (nth ast 2))))
                 ((= head (quote set-el!))
                   (list (quote hs-set-element!) (hs-to-sx (nth ast 1)) (hs-to-sx (nth ast 2))))
+                ((= head (quote view-transition!))
+                  (let ((body (nth ast 2)))
+                    (list (quote hs-view-transition!) (hs-to-sx (nth ast 1)) (if (nil? body) (quote nil) (hs-to-sx body)))))
                 ((= head (quote put!))
                   (let
                     ((val (hs-to-sx (nth ast 1)))