fix(custom-webpack): warn about multiple tsconfigs for custom config

[just-jeb]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #1037

What is the new behavior?

Multiple ts-node registration attempts result in a warning, only first is really registered.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Initially issue was introduced with PR #1035. The original PR aimed to solve ts-node double registration with the same tsconfig and the base assumption was that it would never be called with two different tsconfigs.
That assumption, however, turned to be wrong - in #1037 we see that the usage of @nguniversal/builders results in browser and server builders run in the same process, which effectively throws an exception.
This PR fixes this use case and presents a warning instead of exception.

@GerkinDev FYI.

[autoapproval]

Approved 👍

[GerkinDev]

Thank you. Sorry for the thrown error: at least you know that there are cases where conflicting configuration may happen.

[just-jeb]

It's all good, I reviewed it, remember? 😄
I also presumed it's not a valid use case, but apparently it is.