Update TypeScript to 3.8.1RC

This fixes an issue where property assignments to static methods (please
don't ask me why we have them) would erroneously omit the static method
from the declaration file.

This is fixed in the latest version of TypeScript, most likely as part
of microsoft/TypeScript#36270

DISABLE_THIRD_PARTY_CHECK=Updating TypeScript

Bug: 1011811
Change-Id: I2744ef44c040fdf001f6b0be9e64a164510e0fea
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2064668
Reviewed-by: Jack Franklin <[email protected]>
Commit-Queue: Tim van der Lippe <[email protected]>

Author: TimvdLippe