Improve parser error recovery for misplaced `ref` modifier of a struct declaration

**Case 1:**

**Snippet:**
```cs
ref public struct MyStruct;
```

**Expected:**
`ref` is in the modifiers list for `MyStruct`, error about its misplacement

**Actual:**
`ref` is a part of `RefTypeSyntax` of `IncompleteMemberSyntax` while `StructDeclarationSyntax` starts from a `public` token. The error is 'Type expected' on `public` token

**Case 2:**

**Snippet:**
```cs
partial ref struct MyStruct;
```

**Expected:**
`partial` is in the modifiers list for `MyStruct`, error about its misplacement

**Actual:**
`partial` is its own `IncompleteMemberSyntax`. The error is on `ref` keyword saying about its misplacement instead

Reproduce the two C# snippets from the issue and compare the parser's syntax trees and diagnostics with the expected modifier placement. Trace the parser entry points for struct declarations and incomplete members; done means both snippets retain the intended modifiers on MyStruct and report the misplacement on ref.

Written by the indexing model from the issue text.