Improve CommonJS support

[etal2]

Currently importing the library in a CommonJS Typescript project with the following tsconfig settings:

"compilerOptions": {
    "module": "Node16",
    "moduleResolution": "Node16",
}

fails with this message:

error TS1479: The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. Consider writing a dynamic 'import("csrf-csrf")' call instead.

The solution is to have a separate CommonJS types definition file with a .cts extension.

[psibean]

Interesting, nice catch!

I wasn't aware of the *.d.cts option until now - I rarely work with CommonJS nowadays, so this wouldn't really have been tested. Especially in terms of TypeScript, every TypeScript I touch is ESM, and then only in some cases, the output may be CommonJS.