[Repo Assist] Add isStruct convenience parameter to ProvidedTypeDefinition

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Adds an optional ?isStruct: bool parameter to both public constructors of ProvidedTypeDefinition. This is a Task 10 improvement that makes generating struct value types more discoverable and convenient.

Motivation

Creating a generative struct type provider currently requires knowing to set baseType = Some typeof(System.ValueType) explicitly:

// Before: easy to forget the right base type
let t = ProvidedTypeDefinition("Point", Some typeof(System.ValueType), isErased = false)

With this change, users can use a named parameter that mirrors the existing isInterface, isAbstract, isSealed pattern:

// After: explicit and self-documenting
let t = ProvidedTypeDefinition("Point", None, isStruct = true, isErased = false)
```

The traditional approach still works unchanged (full backward compatibility).

## Changes

| File | Change |
|------|--------|
| `src/ProvidedTypes.fs` | Add `isStruct` to `defaultAttributes`; add `?isStruct: bool` to both public constructors |
| `src/ProvidedTypes.fsi` | Update constructor signatures to expose `?isStruct: bool` |
| `tests/GenerativeStructProvisionTests.fs` | 4 new tests: `IsValueType`, `IsSealed`, `SequentialLayout`, traditional-approach |
| `tests/FSharp.TypeProviders.SDK.Tests.fsproj` | Include new test file |

## Behaviour when `isStruct = true`

1. `TypeAttributes.SequentialLayout` is added to the type attributes (matches what the F# and C# compilers emit for struct types by default)
2. The base type is set to `Some typeof(System.ValueType)` when `None` is passed — this is what drives `IsValueType` detection, the IL type classification, and the assembly writer's `SetParent` call

## Test Status

All 108 tests pass (104 existing + 4 new):

```
Passed!  - Failed: 0, Passed: 108, Skipped: 0, Total: 108, Duration: 5 s

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