Add support for inline Cpp declarations. (#6994)

For #6830, add support for inline C++ fragments as a declaration rather
than as a packaging directive. For now, this uses `inline Cpp
<string-literal>;` as syntax. The prior `import Cpp inline
<string-literal>;` is left alone for the time being. We can decide
separately whether to remove that.

`inline Cpp` requires that there was at least one `import Cpp`. It's not
clear to me if that's the right design long-term, but it seems
reasonable for now.

Assisted-by: Gemini via Google Antigravity

toolchain/parse/handle_decl_scope_loop.cpp

@@ -147,6 +147,9 @@ static constexpr auto DeclIntroducers = [] {
   set_contextual(Lex::TokenKind::Var, ClassContext, NodeKind::FieldIntroducer,
                  StateKind::FieldDecl);
 
+  set(Lex::TokenKind::Inline, NodeKind::InlineIntroducer,
+      StateKind::InlineDeclAfterIntroducer);
+
   set_packaging(Lex::TokenKind::Package, NodeKind::PackageIntroducer,
                 StateKind::Package);
   set_packaging(Lex::TokenKind::Library, NodeKind::LibraryIntroducer,
@@ -226,6 +229,7 @@ static auto ResolveAmbiguousTokenAsDeclaration(Context& context,
         case Lex::TokenKind::Extern:
         case Lex::TokenKind::Fn:
         case Lex::TokenKind::Import:
+        case Lex::TokenKind::Inline:
         case Lex::TokenKind::Interface:
         case Lex::TokenKind::Let:
         case Lex::TokenKind::Library: