clang-format: [JS] do not wrap after `asserts`

`asserts` is a pseudo keyword in TypeScript used in return types.
Wrapping after it triggers automatic semicolon insertion, which
breaks the code semantics/syntax.

`asserts` is different from other pseudo keywords in that it is
specific to TS and only carries meaning in a very specific location.
Thus introducing a token type is probably overkill.

Differential Revision: https://reviews.llvm.org/D100953