Update downlevel-dts for tuple enhancements in 4.0

[RyanCavanaugh]

• Named tuple members (strip the names)
• Variadic tuples (TODO: What do we need to cover?)

[jdmichaud]

I see you were not sure Variadic Typle Types were impacting older TS versions. Do you have a definitive answer on this? Did Variadic Tuples be managed by downlevel-dts? I don't see any change related to this in sandersn/downlevel-dts#38.

[sandersn]

Variadic tuple types definitely do not work in older TS versions. But downlevel-dts is operates only on syntax, and converting a single signature with tuple kinds to a set of overload signature is extremely complex, even with semantic information.

Basically, users of downlevel-dts need to test the generated d.ts files to make sure that the downlevelled code is valid.