TS: do not import with extensions

[Murderlon]

Ref & blocked on: #5258

[aduh95]

Strong -1 from me, we waited for TS to support that before we started migrating, extension-guessing is just adding confusion somewhere it's simply not necessary.

[remcohaszing]

File extensions shouldn’t be used. Instead, the .js extension should be used to import .ts or .tsx files, and module in tsconfig.json should be set to nodenext.

I’m fiddling with this, but I see this causes some problems with Vitest.

[remcohaszing]

After some discussions on Slack and some fiddling, it turns out using the .ts / .tsx extension, or even extensionless or importing a directory containing an index.d.ts file is fine from within a declaration file. That means this change is unnecessary.