Implement static var class fields (#7215)

This adds the `static` token to the lexer and parses it as a modifier.

In check, `FullPatternStack::Kind::FieldDecl` is now used for both
static and non-static vars. Static vars get treated basically the same
as `NameBindingDecl`s.

Global initialization is used for static var initializers. To make the
necessary stack information available to `pattern_match.cpp`, the
`full_pattern_stack` and `decl_introducer_state_stack` are now popped
later in `handle_let_and_var.cpp`.

In lowering, each class's body is checked for `VarStorage` insts and
lowered the same as global vars.

toolchain/check/handle_binding_pattern.cpp

@@ -340,7 +340,7 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
     }
 
     case FullPatternStack::Kind::NameBindingDecl:
-    case FullPatternStack::Kind::FieldDecl: {
+    case FullPatternStack::Kind::ClassScopeVarDecl: {
       if (node_kind == Parse::NodeKind::FormBindingPattern) {
         return context.TODO(node_id, "support local form bindings");
       }