Parser: Don't parse everything as a component

The old parser always try first to find `Foo :=` from the old syntax so it
would error saying it expect an identifier or `:=`
But if we don't find a component, don't try to parse it as a component too early
This way we get better error and better auto-completions

internal/compiler/tests/syntax/parse_error/export0.slint

@@ -2,4 +2,4 @@
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-commercial
 
 export -
-//     ^error{expected Identifier}
+//     ^error{Parse error: expected a top-level item such as a component, a struct, or a global}

internal/compiler/tests/syntax/parse_error/if0.slint

@@ -7,4 +7,4 @@ export TestCase := Window {
     //  ^error{expected ':'}
     //  ^^error{Parse error}
   }
-//^error{expected Identifier}
+//^error{Parse error: expected a top-level item such as a component, a struct, or a global}

internal/compiler/tests/syntax/parse_error/state1.slint

@@ -14,7 +14,7 @@ export TestCase := Rectangle {
             border: 42;
         }
     ]
-//  ^error{expected Identifier}
+//  ^error{Parse error: expected a top-level item such as a component, a struct, or a global}
 
     transitions [
         in pressed: {