Prefer interpretation of scoped as keyword whenever possible

[jcouv]

The following should probably parse as two scoped modifiers.

We should also consider aligning the parser's behavior on scoped.nested a; (parsed as nested type) versus ref scoped.nested b; (error).

This issue is referenced in a couple of tests.

Relates to test plan #59194

        [Theory]
        [InlineData(LanguageVersion.CSharp10)]
        [InlineData(LanguageVersion.CSharp11)]
        public void Local_07_WithInitializer(LanguageVersion langVersion)
        {
            string source =
@"scoped scoped a = default;
";
            UsingTree(source, TestOptions.Regular.WithLanguageVersion(langVersion));

            N(SyntaxKind.CompilationUnit);
            {
                N(SyntaxKind.GlobalStatement);
                {
                    N(SyntaxKind.LocalDeclarationStatement);
                    {
                        N(SyntaxKind.ScopedKeyword);
                        N(SyntaxKind.VariableDeclaration);
                        {
                            N(SyntaxKind.IdentifierName);
                            {
                                N(SyntaxKind.IdentifierToken, "scoped");
                            }
                            N(SyntaxKind.VariableDeclarator);
                            {
                                N(SyntaxKind.IdentifierToken, "a");
                                N(SyntaxKind.EqualsValueClause);
                                {
                                    N(SyntaxKind.EqualsToken);
                                    N(SyntaxKind.DefaultLiteralExpression);
                                    {
                                        N(SyntaxKind.DefaultKeyword);
                                    }
                                }
                            }
                        }
                        N(SyntaxKind.SemicolonToken);
                    }
                }
                N(SyntaxKind.EndOfFileToken);
            }
            EOF();
        }