support `allowArbitraryExtensions`

Enabling [`allowAbitraryExtensions`](https://www.typescriptlang.org/tsconfig/#allowArbitraryExtensions) defines DTS files as `.d.{extension}.ts`.

I'm trying to create strongly-typed JSON files by generating a `.d.json.ts` for each one I need strong types for (until we get [importing JSON `as const`](https://github.com/microsoft/TypeScript/issues/32063)).

tsup doesn't seem to skip these files when generating DTS and instead assumes they're a TS file to be built, but then dies on the `declare const` syntax.

```
DTS Build error
Unexpected token (Note that you need plugins to import files that are not JavaScript)
1: declare const abi: [
^
2: {
3: "type": "function",
```

(oddly this nice error shows up for tsup version ^6.7.0 but not ^8.2.4 and instead only shows "DTS Build error" with no details)

I've tried to work around this with various ignore/exclude options, but nothing seems to work. The TS maintainers suggest that tsup is missing support for the `.d.{extension}.ts` format enabled by `allowArbitraryExtensions` (see conversation in https://github.com/microsoft/TypeScript/issues/49703#issuecomment-2289724261).

Reproduce the failure with a generated `.d.json.ts` file and trace tsup's DTS build path, focusing on how declaration files are identified. Done means files using the `.d.{extension}.ts` format are skipped or handled correctly during DTS generation, with useful error details preserved.

Written by the indexing model from the issue text.