Fix adapter-node build in presence of tsconfig.json

[Conduitry]

Fixes #1667 in a way that (as far as I can tell) does not break compatibility with Node 12.

In Kit apps written in TypeScript, esbuild was seeing the tsconfig.json in the user's app and the target in there was being used since the call to esbuild.build() did not explicitly specify one. This was a problem because the starter app uses es2019, which does not support import.meta.url, and so the references to this (including the ones in the code introduced by the adapter) were being replaced with undefined.

We don't want to tell the adapter to bundle for target: 'es2020' because that would then mean we wouldn't be transpiling features not supported by Node 12. Luckily, esbuild lets us do target: 'node12', which transpiles specifically the features not supported in Node 12. This lets us use ?. (which will get transpiled) and also use import.meta.url (which is left as-is since it is supported natively in Node 12, even though it doesn't support all of ES2020).

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[joidegn]

I was also facing issue #1667 and can confirm that this PR fixes the issue for me.

I have "target": "es2018" in my tsconfig.json