Support TypeScript 6

When using `tsup` with TypeScript 6.x (`typescript@^6.0.0`), the DTS build fails with two issues:

### 1. Deprecated `baseUrl` option

```
error TS5101: Option 'baseUrl' is deprecated and will stop functioning in TypeScript 7.0.
Specify compilerOption '"ignoreDeprecations": "6.0"' to silence this error.
```

**Possible cause:** In `getRollupConfig`, `baseUrl` is explicitly set with a fallback:

```javascript
baseUrl: compilerOptions.baseUrl || ".",
```

### 2. Node types not resolved

```
error TS2591: Cannot find name 'Buffer'. Do you need to install type definitions for node?
Try `npm i --save-dev @types/node` and then add 'node' to the types field in your tsconfig.
```

**Possible cause:** The compiler options merge:

```javascript
if (options.dts) {
options.dts.compilerOptions = {
...tsconfig.data.compilerOptions || {},
...options.dts.compilerOptions || {}
};
}
```

## Workaround

```json
{
"compilerOptions": {
"ignoreDeprecations": "6.0",
"types": ["node"]
}
}
```

## Environment

- `tsup`: 8.x
- `typescript`: 6.x
- `@types/node`: 22.x
- Node.js: 22.x

Start in getRollupConfig and inspect how baseUrl is assigned when generating the DTS Rollup configuration. Then inspect the options.dts.compilerOptions merge and reproduce the DTS build with TypeScript 6.x and @types/node 22.x. Done means the build no longer reports the baseUrl deprecation or missing Buffer types without requiring the documented workaround.

Written by the indexing model from the issue text.