fix: reorder ElementContent so BlockParams wins over AttrNameOnly

The grammar's ElementContent had AttrNameOnly before BlockParams in
the ordered choice, so <Foo as |bar|> was matching 'as' as a
bare attribute name rather than kicking off block params. Put
BlockParams first in the alternation.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: NullVoxPopuli

packages/@glimmer/syntax/pkg/standalone/glimmer_template_parser_bg.wasm

@@ -292,9 +292,10 @@ CloseTag = {
 }
 
 // ElementContent covers attributes, modifiers, splattributes, block params, and comments
+// Check BlockParams first so `as |x|` isn't consumed as attribute name `as`.
 ElementContent = _{
-    WS+ ~ (Splattributes | MustacheComment | AttrModifier | AttrWithValue | AttrNameOnly)
-  | WS+ ~ BlockParams
+    BlockParams
+  | WS+ ~ (Splattributes | MustacheComment | AttrModifier | AttrWithValue | AttrNameOnly)
 }
 
 // TagName unifies all valid angle-bracket tag name forms: