fix: downlevel .d.ts files for compat with older TS versions

[SimenB]

Summary

We shouldn't force consumers to update to TS 3.8. This will downlevel the d.ts files so it works down to TS 3.4.

Fixes #9703
Closes #9704

Test plan

I'll publish a canary and try to build with an older version of TS.

EDIT: Seems to work. This is with typescript@3.4.5

[codecov-io]

Codecov Report

Merging #9705 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #9705   +/-   ##
=======================================
  Coverage   64.94%   64.94%           
=======================================
  Files         288      288           
  Lines       12184    12184           
  Branches     3020     3022    +2     
=======================================
  Hits         7913     7913           
  Misses       3634     3634           
  Partials      637      637           

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[thymikee]

That's gonna add a lot of extra code to download through npm :<

[SimenB]

the definition files aren't that large. But yes, it'll make the package bigger

[SimenB]

@thymikee we could do something to replace the 3.8 definitions with 3.4 compatible ones?

[thymikee]

If that doesn't degrade DX then I'm all in 👍

[SimenB]

It'll degrade DX for consumers, but not for us (as we'll still work on the source). See caveats (semantics): https://github.com/sandersn/downlevel-dts/blob/master/README.md
No big ones though, so possibly worth it?

[thymikee]

Worth it IMO

[SimenB]

PR welcome 😀 Last caveat is that we have to ditch declarationMap, but that's broken anyways since we don't publish source files.

(I personally don't care about install size, so I won't be working it.)

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