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/tokenizer.sx

@@ -131,6 +131,7 @@
     "append"
     "settle"
     "transition"
+    "view"
     "over"
     "closest"
     "next"