Allow for .ts routes file

[Stnaire]

Small changes to allow the generate command to output .ts files.

[bakerkretzmar]

Thanks! This makes sense but it needs a bit more work. Just thinking out loud, I'm happy to make these changes.

• Need to add/update tests.
• Need to check a bit more carefully in the first path normalization because if they pass a .d.ts filename I think that'll make it through and be used by Ziggy to generate the file, which I don't think we want.
• I think the generated file may contain type errors, we might need something like this but I'm not sure and I don't know if there would be other side effects:

declare global {
    var Ziggy: Config;
}