The `ErrorCode` enum in the `error_code.ts` file is governed by public
api guards but the other top-level exports from that file are exempt
from public api documentation and are therefore marked as `@internal`.
However, TypeScript is configured with the `stripInternal` compiler
option such that declarations with `@internal` markers are not emitted
into the `.d.ts` files, but this means that the reexports in the barrel
file end up referring to missing declarations.

The `stripInternal` option is considered internal and its documentation
states to use at your own risk (as per https://github.com/microsoft/TypeScript/issues/45307).
Having the option enabled is desirable for us as it works well for
hiding class fields that are marked `@internal`, which is an effective
way to hide members from the .d.ts file. As a workaround for the issue
with top-level symbols, the declarations with `@internal` markers are
moved to dedicated files for which no public api guard is setup,
therefore allowing their `@internal` markers to be dropped.

Fixes #43097

PR Close #43527