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/modifiers.cpp

@@ -61,6 +61,8 @@ static auto ModifierOrderAsSet(ModifierOrder order) -> KeywordModifierSet {
       return KeywordModifierSet::Extend;
     case ModifierOrder::Decl:
       return KeywordModifierSet::Decl;
+    case ModifierOrder::Static:
+      return KeywordModifierSet::Static;
     case ModifierOrder::Evaluation:
       return KeywordModifierSet::Evaluation;
   }