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/class.cpp

@@ -357,4 +357,18 @@ auto ComputeClassObjectRepr(Context& context, Parse::ClassDefinitionId node_id,
   class_info.complete_type_witness_id = complete_type_witness_id;
 }
 
+auto InNonStaticFieldDecl(Context& context) -> bool {
+  return context.full_pattern_stack().IsCurrentKindClassScopeVarDecl() &&
+         !context.decl_introducer_state_stack()
+              .innermost()
+              .modifier_set.HasAnyOf(KeywordModifierSet::Static);
+}
+
+auto InStaticClassScopeVar(Context& context) -> bool {
+  return context.full_pattern_stack().IsCurrentKindClassScopeVarDecl() &&
+         context.decl_introducer_state_stack()
+             .innermost()
+             .modifier_set.HasAnyOf(KeywordModifierSet::Static);
+}
+
 }  // namespace Carbon::Check