Add type downleveling (typesVersions)

[ecraig12345]

A "good enough" initial solution to ensuring TS 3.9 compatibility is just banning type constructs introduced in 4.0+ (see #18440), but we may want to add actual downleveling in the future. downlevel-dts could help. A bit more discussion on #16101.

[layershifter]

@ecraig12345 is it possible you prioritize this? We have already met this problem, for example, in #19281.

[ecraig12345]

@layershifter I'm not sure if I'll have time to work on this along with other things I'm already supposed to be doing. I also wasn't aware that we had a TS minbar < 4 for converged.

[ecraig12345]

It looks like downlevel-dts is still at the level of a side project at best, though it does support named tuples. A downside is that unless we switched to only shipping a single rollup file, adding a downleveled version of every .d.ts would bloat the package.

The easiest approach might be to add a basic test with TS 3.9 as described in #18440.

[behowell]

@ecraig12345 to follow up on what partner requirements are