support allowArbitraryExtensions

[holic]

Enabling allowAbitraryExtensions 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).

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 microsoft/TypeScript#49703 (comment)).

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