wgsl: Separate struct members with commas

Use of semicolons is still supported, but deprecated.

Also updates the parsing methods for structures to better match the
WGSL grammar.

Bug: tint:1475
Change-Id: I7675ba42c13f91080b0ac173c352e0092021f80b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/84380
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>

test/tint/expressions/zero_init/array/struct.wgsl

@@ -1,8 +1,8 @@
 struct S {
-    i : i32;
-    u : u32;
-    f : f32;
-    b : bool;
+    i : i32,
+    u : u32,
+    f : f32,
+    b : bool,
 };
 
 fn f() {