reference file with extension .ts or filenames containing uppercase letters does not throw Errors

[Uzlopak]

Hi,

we had unfortunately some regressions in mongoose, because I messed up a filename by naming it cursor.ts and not cursor.d.ts. Also there is a bug/feature in typescript that enforces, that filenames of d.ts files should only contain lowercase characters.In both cases tsd does not throw any error.

it would be cool if tsd would recognize this and throw errors.

The bugs we had:
Automattic/mongoose#11469
Automattic/mongoose#11512

Link to the uppercase issue:
microsoft/TypeScript#45096

How I hope we avoid similiar regressions in the future:
Automattic/mongoose#11516